Estimation of fetal age by ultrasound assumes identical biometries for both sexes at identical gestational ages. However, late in the first trimester male fetuses become larger overall than female fetuses, which may introduce a sex bias with clinical consequences. A recent study showed that using due-date estimates based on biparietal diameter from the second trimester increased the post-term male-to-female ratio and the risk of stillbirth among female fetuses born at 43 gestational weeks. We aimed to evaluate whether this increased male-to-female ratio was also present when the due date was based on crown-rump length from the first trimester.
The study population included 3987 women with a certain last menstrual period (LMP), as well as a crown-rump length measured in the first trimester and a biparietal diameter measured in the second trimester. We defined birth after 42 completed weeks estimated by LMP as post-term. Labor was not routinely induced until after 42 weeks. Male-to-female ratios were estimated using logistic regression.
When gestational age was estimated by biparietal diameter, the sex ratio steadily increased from 0.98 (95% confidence interval = 0.87–1.11) in week 40 to 1.54 (1.09–2.17) in week 42. A similar increase did not occur when using certain LMP or crown-rump length.
The use of crown-rump length for the estimation of gestational age is not associated with an increased post-term male-to-female ratio. It can therefore be used for the estimation of due date without risk of the sex bias that occurs when using biparietal diameter in second trimester of pregnancy.