Issues in Pregnancy Dating: Revisiting the Evidence

Linda A. Hunter, CNM, EdD


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

In This Article

Abstract and Introduction


As the end point date in pregnancy, the estimated date of delivery provides guidance for the timing of specific prenatal tests, gauges fetal growth, and informs critical decision making for specific obstetric complications. It is prudent to use the most evidenced-based methods available to accurately determine gestational age. This article explores the accuracy of both menstrual and ultrasound dating techniques and discusses some of the issues and limitations for each method. In addition, a simple formula called the rule of eights can be used to determine a final estimated date of delivery when a discrepancy between menstrual and ultrasound dating occurs.


Establishing an accurate "due date" is one of the most important assessments for a pregnant woman, one that has both social and medical significance. For women and their families, this estimated date of delivery (EDD) represents the long-awaited birth day of their child and is a time frame around which many economic and social activities are planned.[1,2] For obstetric providers, this end point date provides guidance for the timing of specific maternal and fetal testing throughout pregnancy, gauges fetal growth parameters, and provides well-established timelines for specific interventions in the management of prenatal complications.[3,4] In fact, critical decisions, such as preterm labor management, timing of postdate induction of labor, and identification of intrauterine growth restriction are all based on the presumed gestational age of the fetus, which is calculated backwards from the EDD.[3,5,6,7] This article presents an overview of the issues surrounding both menstrual and ultrasound dating and explores the evidence that supports and challenges the accuracy of these methods. Also included is a simple formula for use in practice when discrepancies between menstrual and ultrasound dating occur.


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