Abstract

Objective: Establish normal values for the ratio of the fetal Crown-Rump Length (CRL) to the yolk sac (YS) diameter during the first trimester of pregnancy and determine whether it correlates with gestational age.

Methods: A retrospective analysis of 1453 pregnancies was undertaken from May 1 2012 to June 30 2016 to determine the CRL/YS ratio on uncomplicated pregnancies referred for routine sonographic evaluation between 5 and 11 weeks of gestation at our Maternal Fetal Medicine Unit. Complicated pregnancies, abnormal intrapartum courses and poor fetal birth outcomes were excluded. The mean CRL/YS ratio was established for each gestational age interval and a Pearson coefficient of correlation (r) was calculated with p< 0.05 considered significant.

Results: The CRL, YS diameter and CRL/YS ratio, all significantly correlate with gestational age. The CRL measurements also correlate with the yolk sac diameters for their corresponding gestational ages (r=0.995, p< 0.00001).

Conclusion: This study provides a reference for the relationship between the CRL, the yolk sac diameter and the CRL/YS ratio with gestational age during the first trimester of normal pregnancy. Given the vital importance of the yolk sac to the health of the first trimester fetus, significant changes in the CRL/YS ratio may predict potential complications of pregnancy beyond the first trimester.