Precision of predicting expected date of delivery by ultrasound scan in comparison to last menstrual period

Abstract

INTRODUCTION: Estimation of the expected date of delivery (EDD)is important in management of pregnancy, as many clinical decisions would rely on the estimated gestation of the fetus. In routine clinical practice, this is done by either adding the duration of a pregnancy to the last menstrual period (LMP) or by assessment of fetal biometry by an early pregnancy ultrasound scan (USS). The accuracy of these methods can be assessed either by correlating the dates with people with a know date of conception, such as following IVF, or by study of the correlation with the date of delivery in a cohort of women who goes in to spontaneous labour. In this study we have assessed the accuracy of the two methods by the latter approach. The aim of this study was to assess the agreement of EDD calculated by LMP or USS, to the date of delivery in spontaneous labour. METHODS: This was a retrospective cohort study. 675 patients who had a spontaneous onset of labour were included in the analysis. USS fetal crown rump length was considered for gestational age assessment between 8 and 14 weeks and fetal head circumference considered thereafter. Their agreement of the date of delivery with the estimated expected date of delivery by LMP or USS was assessed. RESULTS: Spontaneous delivery was seen with 6 days of the USS EDD and 7 days of the LMP EDD in 50% of women and within 11days of USS EDD and 15days of LMP EDD among 75% of women. A delivery within 14 date of the EDD was observed in 93.7% when USS EDD was considered and it was only in 86.2% with the LMP EDD (OR 2.40, 95%CI 1.64-3.52). CONCLUSION: This study demonstrate that the USS EDD is more in agreement with the date of spontaneous delivery than the EDD estimated from LMP. This highlights the importance of dating USS in all pregnancies which would be helpful in management of pregnancy in late stages.