Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing.

Amita A Mahendru,Ian B Wilkinson,Sarah Johnson,Christoph Lees,Carmel M Mceniery,Charlotte S Wilhelm-Benartzi

doi:10.1007/s00404-016-4153-3

Abstract

PurposeUnderstanding the natural length of human pregnancy is central to clinical care. However, variability in the reference methods to assign gestational age (GA) confound our understanding of pregnancy length. Assignation from ultrasound measurement of fetal crown-rump length (CRL) has superseded that based on last menstrual period (LMP). Our aim was to estimate gestational length based on LMP, ultrasound CRL, and implantation that were known, compared to pregnancy duration assigned by day of ovulation.MethodsProspective study in 143 women trying to conceive. In 71 ongoing pregnancies, gestational length was estimated from LMP, CRL at 10–14 weeks, ovulation, and implantation day. For each method of GA assignment, the distribution in observed gestational length was derived and both agreement and correlation between the methods determined.ResultsMedian ovulation and implantation days were 16 and 27, respectively. The gestational length based on LMP, CRL, implantation, and ovulation was similar: 279, 278, 276.5 and 276.5 days, respectively. The distributions for observed gestational length were widest where GA was assigned from CRL and LMP and narrowest when assigned from implantation and ovulation day. The strongest correlation for gestational length assessment was between ovulation and implantation (r = 0.98) and weakest between CRL and LMP (r = 0.88).ConclusionsThe most accurate method of predicting gestational length is ovulation day, and this agrees closely with implantation day. Prediction of gestational length from CRL and known LMP are both inferior to ovulation and implantation day. This information could have important implications on the routine assignment of gestational age.

Highlights

Gestational age (GA) assignment has, for many centuries, been based on knowledge of a woman’s last menstrual period (LMP)
Our aim was to estimate gestational length based on LMP, ultrasound crown-rump length (CRL), and implantation that were known, compared to pregnancy duration assigned by day of ovulation
The gestational length based on LMP, CRL, implantation, and ovulation was similar: 279, 278, 276.5 and 276.5 days, respectively

Summary

Introduction

Gestational age (GA) assignment has, for many centuries, been based on knowledge of a woman’s last menstrual period (LMP). The estimated date of delivery (EDD) commonly known as ‘due date’ has traditionally been calculated from this by adding 280 days to the LMP date, or through Naegele’s rule: subtracting 3 from the month and adding 7 days [1] This method of dating a pregnancy is inaccurate where there is not a reliable recollection of LMP, or if. Average cycle variability is approximately 7 days [4, 5] mainly due to variation in length of the follicular phase [6], meaning that ovulation leading to pregnancy has been shown to occur anywhere from day 9–30 of the cycle [5] This introduces a significant error into the use of LMP to assign GA

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