Impact of ovulation and implantation timing on first-trimester crown-rump length and gestational age

Objective To determine the impact of ovulation and implantation timing on first-trimester crownrump length (CRL) and the derived gestational age (GA). Method One hundred and forty-three women who were trying to conceive were recruited prospectively. The timing of ovulation and implantation and the ovulation to implantation (OI) interval were established in 101 pregnancies using home urinary tests for luteinizing hormone and human chorionic gonadotropin. In 71 ongoing pregnancies, GA determined by measurement of fetal CRL at 1014 weeks' gestation was compared with GA based on ovulation and implantation day. First-trimester growth was determined by serial ultrasound scans at 67, 89 and 1014 weeks. Results The median ovulation and implantation days were 16 and 27, respectively, with an OI interval of 11 days. GA estimated from CRL at 1014 weeks was on average 1.3 days greater than that derived from ovulation timing. CRL Z-score was inversely related to OI interval (??=?-0.431, P?=?0.0009). There was no significant relationship between CRL growth rate and the difference between observed CRL and expected CRL based on GA from last menstrual period (??=?0.224, P?=?0.08). Conclusions Early implantation leads to a larger CRL and late implantation to a smaller CRL at 1014 weeks, independent of CRL growth rate. Implantation timing is a major determinant of fetal size at 1014 weeks and largely explains the variation in estimates of GA in the first trimester derived from embryonic or fetal CRL. Copyright (C) 2012 ISUOG. Published by John Wiley & Sons, Ltd.