Student Projects

ACNM Clinical Bulletin: Establishing Estimated Date of Birth (EDB)

Cosette Boone

ACNM Clinical Bulletin: Establishing Estimated Date of Birth (EDB)

Cosette Boone
MW11 Critical Inquiry II
Philadelphia University, December, 2000
Institute of Midwifery, Women & Health, 2000


Establishing the estimated date of birth (EDB) and gestational age of the fetus is of critical importance to midwifery, to obstetrics and to women's health. The gestational wheels used by providers around the country to determine EDB are based on Naegele's rule, the current standard in the United States. Yet this rule, which specifies to add 7 days to a woman's last normal menstrual period (LNMP) and then subtract 3 months from that date, was developed on three assumptions: 1) human gestation is 280 days from the LNMP, 2) the woman has a 28 day menstrual cycle, and 3) ovulation occurs on day 14 of that cycle (1). It is well known though that not every woman has a 28 day menstrual cycle, nor does ovulation always occur on day 14. Research also counters the assumption that human gestation is 280 days. Despite this knowledge, providers continue to use Naegele's rule as the standard seemingly without question.

Systematic errors in the establishment of the EDB may also be connected to the increasing trend of inductions in the United States among both midwives and obstetricians. In 1997, 184 per 1,000 (18.4%) live births were induced, up from 90 per 1,000 (9%) live births in 1989 (2). The single largest category of reasons given for induction of labor is prolonged pregnancy and yet it has been found that approximately 70 percent of inductions done postterm according to menstrual dates are not, in fact, postterm, due to a systematic overestimation of gestational age (3). Additionally, many practitioners are pushing back what used to be defined as postterm. Instead of waiting for 42 completed weeks to recommend induction, now many feel that any time past the EDB is a reasonable choice.

Researchers have attempted to understand this issue by searching for the answers to two questions they have yet to agree upon: 'How long is human gestation?' and 'What is the best method of calculating a woman's EDB?'. One group of scientists challenges the use of Naegele's rule. Among them, Carol Wood (formerly Nichols), CNM, has presented an alternative method of calculating EDB (4). This Clinical Bulletin examines the phenomenon of interest through a presentation of a literature review and its affect on the profession of midwifery and ultimately women's health; it challenges the profession of midwifery to reexamine its unquestioned use of Naegele's rule; and it presents a small descriptive study of statistics from Holy Family Services, a free-standing birth center which has used Wood's rule successfully for over five years.



Establishing the estimated date of birth (EDB) and gestational age of the fetus is of critical importance to midwifery, to obstetrics and to women's health. Every decision the woman and her provider make during a pregnancy is directly related to gestational age. Today, prenatal health care providers use the well-known gestational wheel, based on Naegele's rule, to determine when a woman is likely to give birth. The foundation of this rule, which specifies to add seven days to the date of the first day of a woman's last normal menstrual period (LNMP) and then subtract three months from that date, was based on the following assumptions: 1) human gestation is 280 days from the first day of the LNMP, 2) the woman has a 28 day menstrual cycle, and 3) ovulation occurs on day 14 of that cycle (1). Providers are aware that not every woman has a twenty-eight day menstrual cycle, nor does ovulation always occur on day fourteen. What many do not realize, however, is that there also exists a debate in the literature about the actual length of human gestation and its impact on women's health. It is not agreed how long human gestation really is and more importantly, research has shown that there can be a number of factors that affect the actual EDB. In light of this, it is time to examine an important and concerning trend that is directly related to the establishment of the EDB.

In 1997, 184 per 1,000 (18.4%) live births were induced, up from 90 per 1,000 (9%) live births in 1989 (2). This trend is no different in nurse-midwife attended births. In 1997, the percent of nurse-midwife births that were induced was 14.6, up from 6.5 in 1989; a 124.6% change between the years (5). The single largest category of reasons given for induction of labor is prolonged pregnancy and yet it has been found that approximately 70 percent of inductions done postterm according to menstrual dates are not, in fact, postterm, due to a systematic overestimation of gestational age (3). As early as 1976, this problem existed. A study then found that 70% of births classified as postmature according to LNMP were incorrectly classified and not found to be postmature at all (6). Another study concurred years later by demonstrating that postdate pregnancy occurred in 10.3% of the subjects and yet of that 10.3% only 1.8% of the newborns were discovered to actually be postmature by pediatric assessment for an error of 84% (7). The author points out that if inductions had been routine at 41 weeks instead of after the 43rd week, "nearly one-third of the patients would have been induced" (4). The current standard for postdates induction in the United States is forty-one weeks (8). Thus, the primary problem under investigation lies in the fact that many women today continue to be induced for prolonged pregnancy only to discover that their pregnancy is actually term based on the newborn gestational age assessment. With approximately four million women giving birth each year in the United States, that projects out to approximately 400,000 women being misdiagnosed as being postterm each year. Clearly, there is a problem with how EDB is calculated.


The implications of not establishing a correct due date affects both the midwife and the woman. Most midwives are using Naegele's rule to determine their client's EDB at the initial visit. The due date for the woman symbolizes when her pregnancy will be over and a new child will be born. Every decision regarding the pregnancy is subsequently based on this one calculation, as is all prenatal testing and prenatal education on fetal growth and development and maternal changes. Cultural expectations and individual emotional responses of women to the assignment of an EDB and how these attitudes influence her behavior as the due date approaches and then passes have not been widely studied. If this date is not felt to be accurate, providers turn to an early ultrasound for confirmation. Routine ultrasound increases the cost of healthcare and perpetuates the belief among midwives and women alike, that technology is always needed in order to have a healthy outcome. One of the most troublesome decisions for both midwife and client surrounding the EDB is what to do when the client's pregnancy goes beyond this date.

For the woman, it can mean a series of technological interventions in the name of her and her fetus' well-being, one of which is induction. The dilemma is complicated by the fact that once the decision is made to induce, midwives are faced with the potential of having to manage pregnancies, labors and births with a higher degree of intervention in terms of postdate fetal and maternal evaluation and testing, the use of cervical ripening agents, Pitocin, IVs, continuous monitoring, and restricted mobility. Unfortunately, the implications of misdiagnosing a woman's EDB at that initial visit are not simply an increase in needless induction, but an increase in all the risks that go along with it, including: prolapse of the umbilical cord with artificial rupture of the membranes, fetal death, prolonged labor, prematurity, genital and fetal infection, and the potential failure of the induction itself leading to operative delivery and its risks (9). This scenario sets a woman up for a very different birth experience than the natural one that is not technology dependent. The message to women is clear: their bodies have failed them and therefore need the assistance of technology to function properly.


This Clinical Bulletin is written for the purpose of carefully examining the issue of how to determine a given woman's EDB and the controversy that currently surrounds it. Although scientists have studied and debated this issue since the early 1900s, the standard of care has remained unchanged. It is time that the profession of midwifery critically examine the rising rate of inductions in the United States in its own profession and the possible causes of this disturbing trend. One solution presented below encourages midwives to reexamine the standard use of Naegele's rule, a rule based on many assumptions with absolutely no scientific research to support it (10). An alternative to Naegele's rule, developed by Carol Wood (formerly Nichols), CNM, is presented which, by taking into account various factors, it is argued, more accurately represents the true length of human gestation (4). The use of Wood's rule has the potential of decreasing the amount of inductions done postterm which are not truly postterm. Antidotal evidence from Holy Family Services, a free-standing birth center in Weslaco, Texas, is presented supporting its use. Further research is recommended.


It is important to understand various terminology that will be applied in this discussion:

Preterm gestation is considered to be less than 37 weeks or 259 days from the first day of the LNMP.
Term gestation falls between 37 and 42 completed weeks from the first day of the LNMP or 259 to 293 days.
Postterm gestation is any pregnancy going beyond 42 completed weeks or 294 days or more from the first day of the LNMP (9).
Prolonged pregnancy is a pregnancy that exceeds 40 weeks (280 days) from the date of ovulation or conception. A true prolonged or postterm pregnancy may result in a newborn that is subsequently diagnosed as postmature---a diagnosis that cannot be made prior to birth.
Postdates is used by some authors in place of postterm because it does not imply precise knowledge of fetal maturity or conceptional age, but merely implies that the pregnancy has reached at least 42 weeks gestational age from the first day of the LNMP (4).
Naegele's Rule: Used to calculate the EDB, the rule states to add 7 days to the date of the first day of the LNMP and then subtract 3 months from that date (1).
Wood's Rule: A formula for the estimation of spontaneous delivery which states: 1) add 1 year to the LNMP, 2) then subtract 2 months and 2 weeks for primigravidas, or 3) subtract 2 months and 2.5 weeks for the multipara, then 4) add or subtract as many days as the usual cycle length varies from a 28-day pattern (4).
Length of Human Gestation: The period of time from conception to the birth of the newborn. As defined by Naegele is 280 days from the first day of the LNMP or 266 days from ovulation.
Ultrasound: Ultrasonography is a highly technological piece of equipment used for visualizing soft-tissue structures of the body (in the case of estimating the date of birth, the fetus inside the uterus) by recording the reflection of ultrasonic waves directed into the tissues. An extremely skilled technician is required to operate the transducer. Gestational age is estimated by measuring crown-rump length, biparietal diameter, and in some cases, fetal extremities, abdominal circumferences, orbits, and numerous other anatomic structures (11).


The American College of Nurse-Midwives "...advocates non-intervention in normal processes..." and "supports the use of appropriate technological interventions where the benefits of such technology outweigh the risks" (12). It is not so much a belief that technology is wrong or should never be used, but rather that it has its place and time and purpose.Midwifery education teaches its students to honor low technology alternatives to care. It is a philosophy that emphasizes that the personal time a practitioner gives a client, the touch of a midwife's hands, the questions asked in the history, the conversation shared, are hard to replace. Midwives espouse that the 'gold standard' from which to estimate a woman's date of birth is her LNMP. Yet, research has demonstrated that there appears to be an overestimation of gestational age when the EDB is calculated using Naegele's rule. A disturbing trend noted in midwifery and obstetrics is an increasing number of inductions due to post dates, only to find that the pregnancies are not in fact, postterm. Scientists disagree as to the most effective method for establishing an EDB. Some argue against using the woman's LNMP and state the ultrasound is more accurate, others argue it remains the LNMP. A third argument, central to this issue, is that Naegele's rule needs to be critically examined. In the literature, several authors have offered alternative formulas for calculating EDB, each one honoring the individual characteristics that a woman brings to her pregnancy. Calculating the EDB from a formula requires more personalized time with each woman, taking a thorough history, incorporating that history into the formula, and developing a more intimate relationship with her and the fetus inside her through hands on assessment of maternal and fetal growth patterns. This is exemplary of midwifery's philosophy of advocating a supportive and individualized approach in caring for women.


In the following paragraphs, empirical studies serving as the background for this Clinical Bulletin will be presented. Each chosen article is further divided into one of four groups: 1) those contributing to the current body of knowledge on the length of human gestation, 2) research arguing that a woman's LNMP is still the best gestational age estimator, 3) research countering that assumption, which argues that using ultrasound to calculate gestational age is more accurate than a woman's LNMP, and finally, 4) those authors that propose that rather than criticizing a woman's LNMP, it is in fact, Naegele's rule that should be reevaluated which is the foundation of figuring gestational age based on a woman's LNMP.


Since time began, women have been giving birth and yet one question continues to be debated, "how long is human gestation?". Several research articles were discovered that examined this controversial topic, each challenging Naegele's assumption of 280 days (7, 10, 13, 14, 15). In 1981, one study found that of 297 women with certain dates, the mean gestational age calculated from the LMP was 284.2 days (13). Woods also discovered that the length of human gestation seemed to be greater than 280 days (7). In a retrospective chart review on 175 consecutive patients, it was discovered that the mean length of gestation calculated from the woman's last menstrual period was 283.3 days and that it varied according to parity. Primigravida's estimated to be 287.2 days and multipara's were 280.1 days. One of the largest studies done to date was in 1990 in Sweden to analyze the duration of pregnancy for singleton births between 1976 and 1980 by means of data from the Swedish Birth Registry (14). A total of 427, 581 births were analyzed. In cases of reliable menstrual dates, the average duration from LMP to birth was 281 days (mean), 282 (median), and 283 (mode) days. This study also discovered differences in length of gestation when comparing maternal factors. Mothers aged 35 and over tended to give birth 2 days earlier than those below 35 and multiparous women had shorter gestations than primiparous. It is interesting to note that boys were born earlier than girls and a peculiar seasonal rhythmicity was noted with shorter gestations in the month of December! Another well-known study of 339 births discovered that for primiparas, the median duration of gestation from ovulation to delivery was 274 days (288 days from LNMP) and for multiparas it was 269 days (283 days from LNMP) (10). Three years later, some of the same authors sought to discover which variables influenced length of human gestation (15). Results indicated that maternal parity, age, and race were found to be the most important variables determining the length of human gestation. Multiparous women it was discovered had on average shorter gestational lengths by 3.1 days than primiparous women. Women aged <19 or >34 years also had shorter pregnancies than primiparas as did black women who were found to have shorter gestations by 2.5 days. In women <19 years, the mean length of pregnancy was 270 days from LMP and for women >34 it was 275 days. Several other variables were studied, in all 40, 19 of which were found to be statistically significant in their influence on gestational age (See Appendix A).

Unlike Andersen, Johnson, Barclay & Flora, (13) who used the day of birth, Wood (7) compared the LMP to variables such as gestational age at first audible fetal heart tones, uterine size/fundal height, quickening, ultrasound findings, and the gestational age assessment of the newborn. Some authors would argue that this study compares LMP to variables that are themselves measured based on LMP other than ultrasound (16). The study by Bergsjo, Denman, Hoffman, & Meirik, (14) does not exclude preterm labors, stillbirths past 28 weeks, early neonatal deaths, as well as cases of genetic and teratogenic abnormalities. Every pregnancy between 20 and 48 completed weeks was included. It also included elective cesarean births and induction of labors before spontaneous onset of labor. The authors admit that this may have caused a shift toward a shorter duration overall. Certainly, one of this study's greatest strengths is its sample size and the number of variables with the potential to affect the date of birth and hence length of gestation. One strength of the study by Mittendorf, Williams, Berkey, & Cotter, (10) is that it excluded all complicated pregnancies, unlike Bergsjo, et al., (14). But a criticism is that it established length of gestation by LMP only, with no methods to which to compare it. The authors went on the assumption that one's LMP accurately predicts gestational age based on a study which will be discussed (17). It is important to realize that other studies have found the length of gestation to be actually shorter in certain ethnic groups. For example, Saito, Yazawa, Hashiguchi, Kumasaka, Nishi, & Kato found that Japanese women have shorter gestations, 264.2 days from ovulation to birth (278 days from LMP) (18). In Mittendorf, Williams, Berkey, Lieberman, & Monson, the sample size included 1399 black women, one of the few studies to distinguish race as a variable (15).


There were several studies in the literature which argued that a woman's LNMP remains to be the best indicator of gestational age (13, 14, 17). One study cited frequently in the literature compared ultrasound analysis of gestational age to in vitro fertilization (IVF) pregnancies (17). The sample size included pregnancies from the IVF group which included 34 fetuses (23 pregnancies) and another group of 60 pregnant women who had participated in ultrasound studies of fetal growth. Their discovery was that pregnancies dated using the ultrasound still resulted in 80% more error than a reliable menstrual history. They advised that if a woman knows her LMP within a time-frame of +/- one week, and has regular periods, dates should not be changed unless the discrepancy between menstrual age and ultrasound age is 14 days or more. One criticism of Rossavik and Fishburne's study is the relatively small sample size used, but the authors argue that almost no differences were revealed between the IVF group and the menstrual age group in fetal growth rate which decreases the possibility of finding statistical differences in other and larger groups (17). Two other studies, one by Bergsjo et al. (14) and Andersen, et al. (13) discussed above, also came to the same conclusion---that the LNMP is the most accurate. There is one stipulation: one cannot use an uncertain LMP which would exclude all women becoming pregnant shortly after using oral contraceptive pills, women with unreliable memories of their LNMP, and women without prior bleeding due to other causes like breastfeeding. In these women, gestational age assessment must be based on other parameters. It is important to understand, too, that while the LNMP may be the most accurate standard from which to estimate a woman's EDB, it is only as good as the formula used for this estimation.


To counter the argument for the use of the LNMP, two studies arguing for the use of ultrasound will be discussed here (3, 16). The first study was done to estimate the length of gestation by ultrasound when compared to LNMP (16). The clinical case notes of 476 women were reviewed, 121 of which were excluded due to being abnormal, leaving a total of 355 cases for study. Included were normal pregnancies with spontaneous onset of labor at or after 37 weeks' gestation and labor induced for postmaturity. Routine ultrasound was done between 16 and 21 weeks' menstrual age. Gestational age was determined from the biparietal diameter (BPD) and EDB was assigned according to the "10-day rule" which specifies that if menstrual dates agree within 10 days of ultrasound dates, the former are used; otherwise, the latter are adopted (16). For the purposes of the study, length of gestation was calculated in days from the BPD by computer, and menstrual dates were disregarded. Forty-one women, who by the 10-day rule were thought to be past 294 menstrual days, had labor induced. It was found that of those 41 women whose labors were induced for postmaturity, only 7 were truly postmature when gestational age was figured by ultrasound. This leaves an error of 83% when EDB is calculated by LNMP. It was determined that there is a tendency to overestimate gestation when gestational age is calculated by LNMP and that the "diagnosis of postmaturity on the grounds of menstrual dates can lead to incorrect clinical decisions" (16). The authors concluded that "including menstrual data in the estimation of EDD may lead to high false positives and false negative rates for postmaturity" and that if ultrasound is used to assess gestation, term gestation should be redefined as 264-296 days respectively (16). The second study to argue for the use of ultrasound, did so for the very same reasons. Gardosi, Vanner, & Francis (3) set out to compare gestational dating by ultrasound vs. LMP. Retrospective analysis on 24,675 computer files was done. Gestational dates were calculated between 14 and 22 weeks from BPD. If menstrual dates were outside +/-10 days of the ultrasound scan dates, the ultrasound date was used. Again, it was found that there was an overestimation of gestational age when calculated by LMP accounting for an error of 71.5% in postterm diagnoses. The authors concluded that, "the proportion of pregnancies considered 'post-term' can be reduced considerably by a dating policy which ignores menstrual dates and establishes the expected delivery date on the basis of ultrasound dates alone" (3). Among spontaneous labors, the rate of births occurring after 294 days was 9.5% according to LNMP but only 1.5% by scan. Among labors induced, there was a reduction from 19.3% to 5.5% when scan dates were used.

The argument for the use of ultrasound dating alone is widely addressed in the literature. Each study it seems, like the two discussed above, concludes that by using menstrual dates alone there is a greater percentage of women having induction of labor unnecessarily for postmaturity (19, 20, 21). Even Varney argues that the ultrasound is an accurate measure of gestational age especially in the first half of the pregnancy when it predicts gestational age within 10 days (1). It is not that the ultrasound cannot give an accurate assessment. It is that all these authors are arguing for the adoption of the ultrasound as the standard for determining EDB instead of a woman's LNMP. Midwives must question why this discrepancy in dating exists. Is it that the woman's LNMP is wrong as these articles argue? Or could it be challenged that the standard method used to calculate EDB based on a woman's LNMP is in fact the origin of the problem? From a logical standpoint, it could be argued that the main assumption behind the conclusion that the ultrasound is the better estimator is the argument that the method used to calculate gestational age by LNMP is correct. That method is Naegele's rule. If Naegele's rule, as these authors demonstrate, overestimates gestational age, perhaps it is not the woman's LNMP that is incorrect, but the rule itself which needs to be revised. Thus, the reasoning for encouraging the use of ultrasound in dating pregnancies can be countered. The authors are precisely correct that a problem exists. There solution is to turn to technology and away from nature. The profession of midwifery is in a perfect position here to advocate not for the greater use of ultrasound, but for a more thorough understanding of the standard used in the United States for EDB.


There are many authors that oppose the use of ultrasound for routinely dating a pregnancy and challenge our use of Naegele's rule as the standard in which to measure EDB (4, 10, 22, 23, 24). They state that it is not a woman's LNMP that is the root of overestimating gestational age, but our use of Naegele's rule, based on a woman's LNMP, and all its assumptions as the standard for every woman. Although several authors have posed this challenge throughout the literature, no studies were found testing any new rule in comparison with Naegele's. For the purposes of introducing this debate, the new rules proposed for figuring gestational age based on a woman's LNMP will be discussed briefly here and are presented in Appendix B. It is imperative to understand that none of these rules can be used unless the woman is sure of her LNMP, the length of her cycles, and her contraceptive, sexual, obstetric and gynecological history. As early as 1968, Park reintroduces changing Naegele's rule due to the high incidence of postterm inductions that are not truly postterm (22). He proposes that rather than add only 7 days as Naegele's rule allows, one should add 14 days making the mean 288 days from a woman's LMP rather than 280 days. Carol Wood (4, 23) suggests two different alterations to Naegele's rule. In one study, she proposes that the calculation of delivery date should be done by: 1) add one year to a woman's LMP, then 2) subtract two months, two weeks for primigravidas or 3) subtract two months and 2.5 weeks for the multipara and then 4) add or subtract as many days as the usual cycle length varies from a 28-day pattern (4). This essentially adds 6 days more onto the LMP for the multipara and 10 days more for the primigravida. In another study she simply states that adding 3 days to the EDB calculated by using Naegele's rule "should also increase the accuracy of this important prediction" (23). In 1990, Mittendorf, et al. proposed that when estimating a due date for private-care white patients, one should subtract 3 months from the first day of the LNMP then add 15 days for the primiparas or 10 days for the multiparas, instead of using the common Naegele's rule (10). This essentially adds on 8 more days for the primigravida and three more days for the multipara than Naegele's rule allows. One article quotes, "In 1837 Montgomery stated 'a very common calculation among women themselves is to reckon 42 weeks from the last menstruation or 40 weeks from the middle day of the interval'" (24). They argue that if followed, instead of Naegele's, "much anxiety would be alleviated and a potential stimulus for unnecessary obstetric intervention would be removed". In 1993, Mittendorf, et al. propose an even more detailed formula to calculate gestational age based on several variables that were shown to influence gestational length in their study (15).


Based on the literature reviewed for this study, there were a number of things learned. Although the standard in the United States today continues to use Naegele's rule to calculate the length of human gestation based on a woman's LNMP; Naegele's assumption that 280 days from the LNMP represents the mean is not supported by available data on the length of human gestation. Each study discussed above on the length of human gestation agrees that it is longer than 280 days, but none agree on exactly how long it really is. This discrepancy is most likely due to the number of differences noted in study criteria. Each author studied a different population with different characteristics, all with the potential of possibly influencing the actual day of birth. The fire pushing this debate forward is that research is clearly showing that there continues to be a significant number of women whose pregnancies are diagnosed as postterm, only to discover upon gestational age assessment that they were not postterm. The problem seems to lie in the fact that the calculation of a woman's EDB is being overestimated by Naegele's rule. There has been very little research done on what influence, if at all, assigning an EDB has on a woman. It has been shown that there seems to be an increase in the level of anxiety a woman feels towards wanting her pregnancy to be over once it has reached the due date and that that anxiety continues to rise the further the pregnancy goes beyond that date. Due to this anxiety, a woman is more likely to demand to be induced to end the pregnancy as soon as possible. In an attempt to solve this problem of misdiagnosing the gestational age of the fetus, several studies have been done each presenting its case as to which gestational age measure is actually the most accurate. There are those authors who continue to support the use of using a woman's LNMP as the foundation from which to determine EDB and yet others who counter by arguing that providers should instead rely on technology, specifically the ultrasound, which they believe to be the more accurate measure.

A third faction of the debate represents those who argue that the problem is not the woman's LNMP, but rather the fact that practitioners continue to use Naegele's rule to calculate EDB based on the LNMP! It is the rule they say, not the menstrual period, which is inaccurate. Although various authors have suggested changing the rule to more accurately reflect the true length of human gestation, none have actually tested their proposals. One of the five authors discussed above who challenged Naegele's rule, is nurse-midwife, Carol Wood (4, 7, 23). Her rule: 1) add one year to a woman's LMP, then 2) subtract two months, two weeks for primigravidas or 3) subtract two months and 2.5 weeks for the multipara and then 4) add or subtract as many days as the usual cycle length varies from a 28-day pattern (4); is currently being used routinely at Holy Family Services, a free-standing birth center in Weslaco, Texas with excellent outcomes. Descriptive evidence at this site shows a dramatic difference in postdates inductions compared to the current national average, however, further comparative studies are warranted. {{ADD STATS FROM HOLY FAMILY--I spoke with Cynthia who stated that they have data put together by another midwife scientist years ago between 1993 & 1995 where we can trace Wood's rule outcomes. Since 1995 they have not kept track of which charts used Wood's and which used Naegele's. So without venturing down there to open 300 charts or more, it looks like our data will be inclusive of two years only. But Cynthia suggested commenting that Holy Family continues to use Wood's rule to this date of publication and has had only one newborn assessed with a gestational age of greater than 42 weeks in three years. Cynthia plans on e-mailing me some stats on the last 5 years or so after Thanksgiving (but again, they will not distinguish which charts used Wood's and which Naegele's). The woman who currently has the original disks of the data from 1993-1995 was planning on doing her thesis apparently on this subject. Although I have e-mailed her, I have not had a response. Cynthia said she was on route to Holy Family to be a clinical fellow x 6 months and that I would be able to contact her there after Thanksgiving as well for her data! I would love your help Cindy in analyzing this data once it arrives!}}


Despite all these studies, there is one crucial piece missing in this puzzle. How do other proposed rules for calculating EDB from a woman's LNMP compare with the current standard in the United States today, Naegele's rule? Currently, there are no quantitative studies examining these rules, especially rules that take into account all the individual factors that influence the length of a woman's pregnancy. Also lacking in the literature is the effects of societal and cultural expectations around a woman's due date. What are the implications on a woman who's pregnancy has been diagnosed as 'prolonged'? What about the midwife? How are midwives coping with this dilemma and what does their counseling to women entail?


In light of the promising descriptive statistics from Holy Family, it is in the best interests of the profession of midwifery to critically examine the way we routinely figure a woman's EDB and what affect, if any, a new rule would have on the current increasing rate of inductions. It is time that Naegele's rule, with all its assumptions and no research to support its standard use for every woman, is challenged. Preconceptionally, midwives can begin to educate women of the importance of keeping a thorough menstrual history, possibly even educating women to be more aware of their time of ovulation and potential moments of conception. Recommending they keep a journal may be a start. Time with the client in the initial prenatal interview is imperative to collect a thorough menstrual, contraceptive, sexual, gynecological, and childbearing history. The time a practitioner gives to hearing a woman's story and doing a thorough physical assessment of her and her fetus throughout her pregnancy will be invaluable in terms of determining a fetus' gestational age. Review the research that is available which discusses all the factors that play into how long a given woman's gestational period will be, and keep these factors in mind when the 'due date' is put in the chart.

It is interesting to note that only one study was found which examined women's attitudes to a proposal of conservative management of prolonged pregnancy (25). It was discovered, despite education on risks of induction, that when given an option between induction at 42 weeks or a watch and wait approach up to 44 weeks, the majority of women wanted to be induced. The main reason cited at 37 weeks for wanting to be induced was the woman, "could not stand the thought of being pregnant for more than 42 weeks". Other reasons cited were, "no benefit in waiting", "no risks involved in having labour induced", "no member of the family available after 42 weeks gestation", and "concern regarding fetal size". To help decrease the anxiety women have around their due date, it is possible that midwives can help women by encouraging them not to focus on a particular day, but rather to celebrate and look forward to their 'birth month'. Education during the prenatal care period is critical and should include a discussion of the current problem of the trend of rising rates of induction and the current debate regarding how providers determine a woman's EDB.


One mystery that still remains in our world is the length of human gestation and all the variables that affect that length for each given woman. Rather than getting caught up in how technology might be able to solve this dilemma, why not just simply trust that the design of the woman's body is inherently meant to work? It is important to be aware of what other midwives around the country are doing to tackle the problem of the rising rate of inductions in the profession due to a woman's pregnancy going post dates. Different ways of configuring EDB are being tried, other than Naegele's rule, that are proving to be quite promising. In light of the statistics from Holy Family, each midwife around the country should begin to critically examine how he/she comes up with this important date, a woman's due date, and begin looking at how it could be done differently to more accurately reflect each individual woman's true length of gestation. Remember that Wood's rule has not been proven to be better than Naegele's, nor would it be prudent to begin endorsing it without further research; but it is imperative that we recognize the current trends on induction, and the debate in the literature over the standard use of Naegele's rule.

Without further research, this issue and all its implications will never be fully understood. Above all, women have a right to know about the current debate over calculating her due date and all the factors that influence her particular EDB and length of gestation. The profession of midwifery is in a prime place to promote a supportive and individualized approach to determining an EDB and respecting the inherent design by nature of a woman's body. The search for a more accurate rule just may prove to decrease the number of ultrasounds for dating and midwifery's rate of inductions. One interesting thought, is that while this debate is going on among researchers, midwives and physicians; women continue to labor and birth, and their babies continue to be born on their own schedule---doing what the body is meant to do regardless of whether or not scientists figure out how to determine exactly when it is going to happen. It is highly likely that it will never be known to the day when birth is going to occur for each individual woman, for it is still unknown why labor even begins, but the pursuit for more in-depth knowledge is not simply for the profession of midwifery, but more importantly, for the betterment of women's health around the world.


1. Varney, H. (1997). Varney's Midwifery (3rd ed.). Boston: Jones and Bartlett Publishers.
2. ACOG Committee on Practice Bulletins---Obstetrics. (1999, November). Induction of labor. ACOG Practice Bulletin(10), 1-5.
3. Gardosi, J., Vanner, T., & Francis, A. (1997, July). Gestational age and induction of labour for prolonged pregnancy. British Journal of Obstetrics and Gynaecology, 104, 792-797.
4. Nichols, C. W. (1985a). Postdate pregnancy: Part II. clinical implications. Journal of Nurse-Midwifery, 30(5), 259-268.
5. Curtin, SC. (1999). Recent changes in birth attendant, place of birth, and the use of obstetric interventions, United States, 1989-1997. Journal of Nurse-Midwifery, 44(4), 349-354.
6. Boyce, A., Mayaux, J. M., & Schwartz, D. (1976). Classical and "true" gestational postmaturity. American Journal of Obstetrics and Gynecology, 125(7), 911-914.
7. Nichols, C. W. (1985b). The Yale nurse-midwifery practice. Journal of Nurse-Midwifery, 30(3), 159-166.
8. Hannah, M.E., Hannah, W.J., Hellmann, J., Hewson, S., Milner, R., Willan, A., & the Canadian Multicenter Post-Term Pregnancy Trial Group. (1992). Induction of labor as compared with serial antenatal monitoring in post-term pregnancy. The New England Journal of Medicine, 326(24), 1587-1592.
9. Oxorn, H. (1986). Oxorn-Foote: Human Labor & Birth (5th ed.). Norwalk, CT: Appleton & Lange.
10. Mittendorf, R., Williams, M. A., Berkey, C. S., & Cotter, P. F. (1990, June). The length of uncomplicated human gestation. Obstetrics & Gynecology, 75(6), 929-932.
11. Fischbach, F. (1996). A manual of laboratory & diagnostic tests (5th ed.). Philadelphia: Lippincott.
12. American College of Nurse-Midwives. (1997, August). The appropriate use of technology in childbirth. Postition Statement.
13. Andersen, F., Johnson, R. B. T., Barclay, L. M., & Flora, D. J. (1981). Gestational age assessment: I. Analysis of individual clinical observations. American Journal of Obstetrics and Gynecology, 139, 173-177.
14. Bergsjo, P., Denman III, W. D., Hoffman, J. H., & Meirik, O. (1990). Duration of human singleton pregnancy: A population-based study. Acta Obstet Gynecol Scand, 69, 197-207.
15. Mittendorf, R., Williams, M. A., Berkey, C. S., Lieberman, E., & Monson, R. R. (1993). Predictors of human gestational length. American Journal of Obstetrics & Gynecology, 168(2), 480-484.
16. Mongelli, M., & Opatola, B. (1995). Duration and variability of normal pregnancy: Implications for clinical practice. Journal of Reproductive Medicine, 40, 645-648.
17. Rossavik, K. I., & Fishburne, I. J. (1989). Conceptual age, menstrual age, and ultrasound age: A second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period. Obstetrics and Gynecology, 73(2), 243-249.
18. Saito, M., Yazawa, K., Hashiguchi, A., Kumasaka, T., Nishi, N., & Kato, K. (1972). Time of ovulation and prolonged pregnancy. American Journal of Obstetrics and Gynecology, 112(1), 31-38.
19. Kramer, M., McLean, F., Boyd, M., & Usher, R. (1988). The validity of gestational age estimation by menstrual dating in term, preterm, and postterm gestations. JAMA, 260(22), 3306-3308.
20. Backe, B., & Nakling, J. (1994). Term prediction in routine ultrasound practice. Acta Obstetricia et Gynecologica Scandinavica, 73, 113-118.
21. Mongelli, M., Wilcox, M., & Gardosi, J. (1996). Estimating the date of confinement: Ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol, 174(1), 278-281.
22. Park, L. G. (1968, December). The duration of pregnancy. The Lancet, 1388-1389.
23. Nichols, C. (1987). Dating pregnancy: Gathering and using a reliable data base. Journal of Nurse-Midwifery, 32(4), 195-204.
24. Saunders, N., & Paterson, C. (1991). Can we abandon Naegele's rule? The Lancet, 337, 600-601.
25. Roberts, J. L. & Young, R. K. (1991). The management of prolonged pregnancy---an analysis of women's attitudes before and after term. British Journal of Obstetrics and Gynaecology, 98, 1102-1106.



Shorter Gestations
<19 or >34
Black women (2.5 days)
lower educational level
excessive consumption (each cup/day shortened gestation by 0.39 days, thus if a woman drinks 5/day, 0.39 x 5 = 1.95 days shorter gestation)
multiparous (3.1 days)
Incompetent Cervix
DES Exposure
Hx of Miscarriage
Hx of Stillbirth
Obstetric Complications
Any Vaginal Bleeding during the pregnancy
Placenta Previa
Abruptio placenta
Premature Rupture of Membranes
Pregnancy Induced Hypertension
(Mittendorf, et al., 1993).


Park, 1968
Add14 days to the woman's LNMP and then subtract three months; making the mean 288 days rather than 280
Nichols, 1985a
1) add one year to a woman's LNMP, then 2) subtract two months, two weeks for primigravidas or 3) subtract two months and 2.5 weeks for the multipara and then 4) add or subtract as many days as the usual cycle length varies from a 28-day pattern. This adds 6 days more onto the LNMP for the multipara and 10 days more for the primigravida than Naegele's rule allows.
Add 3 days to the EDB calculated by using Naegele's rule
Mittendorf, et al., 1990
Subtract 3 months from the first day of the LNMP then add 15 days for the primigravidas or 10 days for the multiparas, instead of Naegele's rule. This adds 8 more days for the primigravida and 3 more days for the multipara.
Saunders, & Paterson, 1991
Calculate 42 weeks from the last menstruation or 40 weeks from the middle day of the interval.


Journal of Midwifery & Women's Health: CLINICAL BULLETIN

Not unlike the clinical bulletins put out by ACOG, the journal's bulletin is distinct in that it focuses on midwifery management, not medical management. There are three types of bulletins, each with a different focus: 1) care for women at risk, 2) advanced practice skills, and 3) low technology alternatives. The intention of the ACNM in developing these documents is to "support midwifery practice through the presentation of evidence supporting midwifery management, guidelines for practice, and/or a discussion of emerging issues in women's health care"
(L. Summers, personal communication, October 13, 2000). In speaking with Dr. Summers at the ACNM on one of my fact-finding missions, she suggested I turn this assignment into a Clinical Bulletin for the journal. Although it is not yet set in stone that this is what I will do, I thought I should present this information for students' future knowledge. I would imagine that my topic would be submitted under the category of 'low technology alternatives' as a way of emphasizing how midwives could significantly impact the number of inductions for post-date pregnancies.

Each clinical bulletin addresses one issue, presenting its scientific or historical basis and its significance to midwifery and women's health. The guidelines ask for a presentation of any evidence supporting recommendations for clinical practice. As no formal studies have yet been done comparing Nichol's rule with Naegele's, I felt I could not write a clinical practice guideline stating Nichol's is the best way to go. There is however, years of clinical practice evidence from Holy Family which has been using Nichol's rule with no problems. This data, combined with current research presented on length of human gestation, will make a strong argument for reexamining the standard of care in the United States today and could be presented nicely following the guidelines for writing a clinical bulletin.

Authors must use the current format of the Journal of Midwifery & Women's Health for text and references which follows the "Uniform Requirements for Manuscripts Submitted to Biomedical Journals", published in the Journal of Midwifery & Women's Health, volume 42, number 3, May/June 1997. Two typed, double-spaced, single sided copies are required, as well as a diskette in Word Perfect format. A single copy of each reference cited must accompany the text.


November 20, 2000

Lisa Paine, CNM, DrPH, Editor-Elect
Journal of Midwifery and Women's Health
Department of Maternal Child Health
Boston University School of Public Health
715 Albany Street, T5W
Boston, MA 02118-2526

Dear Ms. Paine:

Enclosed are two copies of a manuscript entitled: ACNM Clinical Bulletin: Establishing Estimated Date of Birth (EDB); a signed copy of the Author Agreement and the Author Checklist, a consent form from Holy Family Birth Center to use their data on Wood's rule, and permission from Carol Wood, CNM, Ed.D for use of her rule. This manuscript is my final project in the Masters of Midwifery program at Philadelphia University. During the research phase, I spoke with Dr. Lisa Summers who suggested I turn the manuscript into a Clinical Bulletin for potential publication. My professor, Cindy Farley, CNM, PhD, has been an integral part of the editing process and analyzing the retrospective data presented from Holy Family Birth Center.

Please direct all correspondence or questions concerning this manuscript to me at the following address:

Cosette Boone, CNM, MS
1818 Pleasant St. #4
Des Moines, IA 50314

Thank you for your consideration of this paper. I would be honored to edit it in any manner you may recommend.


Cosette Boone, CNM, MS


To: Cosette Boone/Institute

Subject: Re: Cosette's Chapter 3---comments welcome

First, I want to say, that your paper is fascinating and I, too, found it hard to tell what could be edited from it: I considered two small paragraphs 1) page 3 maybe leave out the definition of ultrasound, and possibly preterm labor;
and 2) page 4 the Review of literature section may not be needed.

I understand the problems of misdiagnosing a postterm baby, but I find it totally arrogant to think that we are able to define a definite length of gestation. Do we know the exact length of a human life, the exact days of puberty? etc., I like your idea of Estimated Month of Birth. But WOW!! What a job that would be to change our current mode of thinking, of a Due Date. As you said we would be educating the whole American macrosomic infants?


PS Have a good holiday.

MY COMMENTS: I did not incorporate the advice at this time. Although I do wonder about the definition section. Is it all truly needed?

To: Cosette Boone/Institute@Institute

Subject: coment

hi cosette, just read your paper and found it facinating and very well done. having used the nicol's dating at holy family, i have seen the concept inaction , as have you. i might mention in your section about the holy family stats that there was not one post mature baby at holy family, at least in the year that i was there. well, i was there for 6 months , but i am referring to the year of which thise months were a part. i believe there may have been one post mature baby in the period of time since i left. you may have been there then.

bravo. great paper, well done and good luck with the publishing.

MY COMMENTS: I did incorporate into the paper that there has only been one postmature newborn in three years. I confirmed this data with Cynthia at Holy Family.

Sarah's comments:


Since time began, women have been giving birth and yet one question continues to be debated, "how long is human gestation?". Several research articles were discovered that examined this controversial topic, each challenging Naegele's assumption of 180 (you mean 280, right?) days.

Under US should be the standard:

Were these studies looking at BPD's, Ac's, FL's only? Or did they take CRL into consideration? I would think that the second tri measurements have more room for error. I agree that it does call Naegles rule into question.

In the graph:

excessive consumption (each cup/day, right? shortened gestation by 0.39 days, thus if a woman drinks 5/day, 0.39 x 5 = 1.95 days shorter gestation)

MY COMMENTS: I went back and made the small corrections and then rewrote the ultrasound section a bit to clarify some of the confusion. It is still a work in progress.
. . . . . . .