Issues in Pregnancy Dating: Revisiting the Evidence

Menstrual Dating

Human gestation has been traditionally thought to last a mean of 266 days from the date of conception or 280 days (10 lunar months) from the first day of the last normal menstrual period.^[7,8,9] Because the typical menstrual cycle lasts 28 days with ovulation presumed to occur around day 14, the first day of the last menstrual period (LMP) has been used as the primary pregnancy dating marker for more than 200 years.^[10] Frederich Naegele, a 19th-century obstetrician, is credited with developing a simple calculation to determine the EDD by adding 7 days to the first day of the LMP and then subtracting 3 months.^[10] This calculation, commonly referred to as Naegele's rule, establishes an EDD that is more or less 280 days from the LMP depending on which calendar months are involved.^[10,11] Because ovulation is not an easily observed event, using Naegele's rule provides an indirect measure of conception and remains the current standard for calculating the EDD based on the first day of the LMP.^[9,12] The reliability of this method depends on several factors: the woman's accurate recall of her LMP; the regularity of her cycles; the presence of early or light bleeding; and other factors, such as oral contraceptive use or breastfeeding that could influence ovulation timing.^{[5,7,9,13,14]}

There can also be some variation in the actual timing of ovulation, even in the presence of a seemingly normal 28-day menstrual cycle.^[15,16,17] For example, Baird et al.^[15] examined the ratio between changes in urinary estrogen and progesterone that normally occur at the time of ovulation. In their assessment of 498 normal menstrual cycles, they reported a range of 7 to 19 days for the luteal phase, with only 10% of women ovulating on day 14.^[15] Similar findings were later reported by Wilcox et al.,^[16] who additionally stated that only 30% of women with normal 28-day cycles are "fertile" between days 10 and 17. More recently, Nakling et al.^[17] found substantial biologic error between the onset of the menstrual period, the timing of ovulation, and the actual start of pregnancy, concluding that fertilization could potentially occur any time in the menstrual cycle. Interestingly, no recent studies were found examining the length of human gestation; however, past sources have reported data that suggest human gestation lasts up to 7 days longer than the presumed 280 days.^[7,8] These results support the findings of Baird et al.,^[15] Wilcox et al.,^[16] and Nakling et al.,^[17] which suggest that Naegele's rule does not correlate reliably with the date of conception despite the accuracy of the reported LMP.

Prem's Rule

In the lay literature, an obstetrician from the University of Minnesota is credited with developing "Prem's Rule,"^[18,19] which attempts to more accurately calculate the EDD from conception by using basal body temperature charts. In this method, 7 days are subtracted from the first day of thermal shift and 9 months are then added to obtain the final EDD.^[18,19] While no references to this method were found in the medical or nursing literature, most authors agree that using actual conception dates, especially when using insemination or in vitro fertilization techniques, are far more likely to yield the most accurate EDD (Table 1).^[9,12,14,20]

Unfortunately, because more than 50% of pregnancies are unintended, the majority of women are not tracking their ovulation and must rely on recall of their LMP to calculate the EDD.^[12]

Errors in LMP Recall

Even with the most careful questioning by well-intentioned nurses, midwives, and physicians, there can be considerable errors in a woman's LMP recall, further adding to the dubiousness of this time-honored rule.^[4,14,21,22] For example, in a review of more than 43,000 birth records, Walker et al.^[4] found that women were more likely to report their LMP as occurring on one of seven preferred days with "15" being the most reported LMP date across all socioeconomic and ethnic groups. The authors^[4] attributed this unexpected finding to recall "rounding," which would most likely lead to overestimating the gestational age, especially when compared to early ultrasound dating. Interestingly, Savitz et al.^[14] found a similar trend towards preferred LMP dates, also noting "15" to be the most commonly selected number. This tendency towards preferred numbers further increased the discordance with ultrasound dating -- a factor Savitz et al.^[14] contend could lead to inaccurate assignment of a more advanced gestational age and subsequent misclassification of postterm pregnancy. Furthermore, Wegienka and Baird^[22] prospectively studied women's recall of their most recent LMP and found their study participants' memories became significantly less accurate over time, even by as little as 3 weeks from the actual onset of their last menses.

The Role of Pregnancy Wheels

Another more frustrating aspect to establishing an accurate EDD is the commonplace use of pregnancy wheels. These wheels are manufactured primarily by drug companies and are used extensively in obstetrics to calculate both the EDD and gestational age.^[11] Despite their convenience and widespread use, there can be up to a 5-day difference between different wheels, and surprisingly few research studies have addressed this inconsistency, with none showing any evidence of manufacturer quality control.^[11,23] In addition, pregnancy wheels typically do not correlate with Naegele's rule.^[11] McParland and Johnson^[23] also reported the added likelihood of inaccurate readings caused by misalignment from the central mounting on the wheel and the presence of unevenly spaced lines, making these wheels notoriously hard to read. It is unclear why the continued use of pregnancy wheels is still so pervasive, especially because there are now several readily available computer-based systems using actual calendar dates that are free for use by anyone with a computer that can use the software.^[11] Ross^[11] concludes that until a more acceptable and consistent method is developed to determine gestational age, obstetric providers should be aware of these potential inaccuracies and carefully evaluate the accuracy and reliability of their current dating techniques.

Naegele's Rule Revisited

One last caveat in regard to the issues raised with menstrual dating can be found in Basket and Naegele's^[1] review of Frederich Naegele's original 19th-century German obstetrics textbook. These authors^[1] discovered that the centuries-old Naegele's rule was in fact not Naegele's original idea but based on the writings of Hermann Boerhaave, an 18th-century professor of botany and medicine in the Netherlands. Naegele credited Boerhaave with developing the formula to calculate the delivery date from the last menses, which some historians believe Boerhaave loosely based on observations found in the Bible that human gestation lasted 10 lunar months (280 days).^[1] Later textbooks of obstetrics and gynecology in the early 20th century began crediting Naegele with this rule, and it has been unquestionably handed down as the standard for pregnancy dating ever since.^[1]

What remains unclear in the translation and review of both Naegele's and Boerhaave's writings, however, is whether the calculation was meant to start from the beginning or the end of the last menses.^[1] Defining Naegele's rule as specifically starting from the first day of the LMP was thought to have originated in some of the early American obstetric textbooks and was never specified as such in Naegele's original writings.^[1] In response to this concern, Baskett and Naegele^[1] suggest that Naegele and Boerhaave were possibly misinterpreted and that the calculation might have more reliability when compared to ultrasound dating if counting began at the end of the menses or 10 days were added to the first day of the LMP instead of the standard 7 days used today when doing the calculation. As technology advances and more and more women receive ultrasound testing in pregnancy, it remains to be seen whether Naegele's rule, regardless of its original inception, will continue to stand the test of time.

Proposed Solutions

Not surprisingly, several authors support eliminating LMP dating altogether, stating that the margin of error on even the most accurate LMP recall, regardless of how the EDD is calculated, is still unacceptable in modern times.^[1,2,13,14] Arguing in favor of ultrasound dating, they also reason that having the most accurate EDD will reduce anxiety for the pregnant woman and her family, especially in the postterm but undelivered clinical scenario.^[1,2,13] Katz et al.^[2] further rationalize this stance by recommending that pregnant women be given an "assigned week of delivery" in lieu of the EDD, stating that this would eliminate the reassignment of due dates late in the pregnancy to correct or meet patient's expectations. Although the title of their commentary suggests that Katz et al.^[2] at times approach these issues "in jest," it seems feasible to consider a composite gestational age assessment based on all available methods. At the very least, they make a valid point in regard to the social significance of the EDD for pregnant women compared to the clinical and legal implications that gestational age holds for obstetric providers in the management of pregnancy.^[2]

Table 1. Methods for Determining Menstrual Age
Clinical or Sonographic Parameters	Variability Estimates (2 SD)
In vitro fertilization^a	±1 day
Ovulation indication^a	±3 days
Artificial insemination^a	±3 days
Single intercourse record^a	±3 days
Basal body temperature record^a	±4 days
First-trimester physical examination	±2 weeks
Second-trimester physical examination	±4 weeks
Third-trimester physical examination	±6 weeks
First-trimester sonographic examination (CRL)	±8% of the estimate
Second-trimester sonographic examination (HC, FL)	±8% of the estimate
Third-trimester sonographic examination (HC, FL)	±8% of the estimate

Table 2. Methods for Determining Menstrual Age
Calculation	Example
1. Double-check and recalculate the LMP due date (ideally with a computer)	LMP 4/18/08 → EDD 1/23/09
2. Calculate the number of days difference between the LMP and US due dates	US 8/15/08 20 + 2 wks → US EDD 1/10/09 (13-day difference)
3. Convert the GA from the earliest US to days	GA of 20 wks 2 days = 142 days (20 x 7 + 2)
4. Multiply this number by 0.08 for the 8% MOE	142 days x 0.08 = MOE 11.36
5. If the difference is greater, use the ultrasound date	Difference > MOE = US date
6. If the MOE is greater, use the LMP date	Difference < MOE = LMP date
7. Document the calculation in the chart with the "final EDD"	Final EDD 1/10/09

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