Issues in Pregnancy Dating: Revisiting the Evidence

Linda A. Hunter, CNM, EdD


J Midwifery Womens Health. 2009;54(3):184-190. 

In This Article

A Simple Solution to Dating Discrepancies: The Rule of Eights

One of the more frustrating situations occurs when there is a significant difference between a late second- or early third-trimester ultrasound EDD and the reported LMP due date. This scenario is often encountered when the woman presents late to prenatal care or did not have an earlier ultrasound assessment. In this case, the accuracy of the LMP and the accuracy of the ultrasound come into question. How should obstetric management proceed in this scenario? Which due date should prevail?

In today's ultrasound-savvy environment, ultrasound biometry performed by an experienced provider has a fairly consistent 8% margin of error at any gestation (Table 1).[20,34,35] Therefore, one can potentially calculate and compare this margin of error against a dating discrepancy at any point in pregnancy. Because ACOG[27] has established guidelines for managing dating discrepancies up until 20 weeks' gestation that are not based on this evidence, using the 8% margin of error may be more useful in later gestations when there is discordance between menstrual and ultrasound dating.

Accordingly, one busy clinic service adapted this margin of error into a standardized calculation, commonly referred to as the "Rule of Eights" (Table 2). Although not published as an official dating standard, this easy to use formula was originally derived from Hadlock et al.'s[36] regression analysis using different combinations of fetal growth parameters to establish gestational age. This landmark study presented the first of many regression tables published by Hadlock et al. in their ongoing research of the most accurate fetal growth estimates.[29,34,35,36] What is most interesting about Hadlock's tables are that the results typically report 95% confidence intervals with standard deviations as ± weeks gestation. These results can be easily translated to a percent of the estimate once converted to days' gestation, a mathematical distinction Hadlock himself clarifies and advocates in a later publication.[35] Consequently, the 8% margin of error represents a simple conversion that more easily lends itself to managing dating discrepancies found in clinical practice.

Calculating the Rule of Eights

The steps for approaching a dating discrepancy using the Rule of Eights can be found in Table 2. In using this approach, it is first assumed the patient has initially reported a reliable LMP date and that her EDD has been calculated accordingly. Because the 8% margin of error has been found to be consistent throughout gestation (Table 1), one could technically use the Rule of Eights in any trimester when a dating discrepancy is encountered.

Current ACOG[27] guidelines recommend changing the EDD when a first-trimester ultrasound differs more than 7 days from the LMP date or more than 10 days between 12 and 20 weeks' gestation. Interestingly, Hadlock's[35] data indicate that 8% in the first trimester would represent ± 5 days difference and ± 11.2 days difference at 20 weeks' gestation. Regardless of which guideline is used in early pregnancy, a more challenging scenario occurs in later trimesters, where the accuracy of ultrasound will yield a progressively widening variance. By applying the Rule of Eights in these latter situations, discrepancies can be easily resolved and a final EDD established. Any discordance with future ultrasound measurements would be indicative of possible growth abnormalities.

One important advantage of using this standardized approach has been the uniformity among providers in establishing a "final" EDD that leaves little room for diverging opinions in the face of obstetric complications. In a busy tertiary care hospital with a high volume of obstetric patients, consistency is crucial, especially when specific treatments are recommended that are based on the most accurate gestational age assessment. In addition, using the Rule of Eights in a teaching hospital is a great tool for residents and medical students who not only learn to critically examine any and all dating criteria for each patient, but also to document clearly in the prenatal chart how the EDD was established. Consequently, everyone is speaking the same language in regards to gestational age and decisive, evidence-based management plans can then be carried out with confidence.


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