Issues in Pregnancy Dating: Revisiting the Evidence

Conclusion

This article's primary aim was to present some of the evidence that supports and, in some cases, challenges the reliability and feasibility of menstrual and ultrasound dating in pregnancy. Despite the best efforts of many researchers, human variation abounds and accurately predicting the day of delivery continues to elude us. Of course, the EDD has always been an "estimate" and was never intended to predict the actual day of birth. Unfortunately, when facing obstetric complications, one cannot medically (or legally) say, "the gestational age is around 32 weeks." So we are challenged with converting our best guess into what seems, in some cases, like a firm line in the sand. The Rule of Eights has facilitated this translation at one institution and has greatly improved consistency in the identification of growth abnormalities and the management of pre- and postterm patients. We are all fortunate to have the availability and advancing technology of ultrasound as an adjunct to time-honored LMP dating that in most cases is still considered a reliable indicator of the EDD. But we must continue to exercise caution in relying on a single method in every case. Our best approach lies in understanding the inherent inaccuracies of some of the methods we have relied upon to calculate the EDD, using the most evidence-based techniques available, and including our patients in all discussions regarding how their final EDD was determined. We must also continue to critically examine the evidence with regard to the potential limitations and possible overuse of ultrasound. Last and most importantly, once a final EDD has been established, it should remain the set point clinical marker for all obstetric decision making and any future assessments of adequate fetal growth.

I would like to acknowledge Dianne Reynolds, CNM, and Allan Nadel, MD, for their assistance in locating the origin of the Rule of Eights formula.

J Midwifery Womens Health. 2009;54(3):184-190. © 2009  Elsevier Science, Inc.