Early first-trimester crown-rump length measurements in male and female singleton fetuses in IVF pregnancies

Objectives: The generally higher birth-weight of male newborns compared to female newborns is attributed mainly to intrauterine exposure to testosterone. We aimed to determine if crown-rump length (CRL) differs between male and female fetuses early in the first trimester. Methods: A retrospective cohort study of 333 women with nondiabetic singleton IVF pregnancies attending a single university-affiliated tertiary medical center in 2000-2007 who underwent CRL measurement before 50 days of pregnancy (date of measurement minus oocyte retrieval date plus 14 days). Data on pregnancy outcome, including fetal sex, were collected by routine follow-up telephone interview and combined with the delivery data. Results: There were 169 female and 164 male fetuses according to the reported fetal sex at delivery. Most of the CRL measurements (68.7%) were performed at an actual gestational age of 43-45 days. On linear regression analysis, male fetal sex was a significant (p = 0.011) predictor of larger CRL: CRL (mm) = -23.851 + GA (days) × 0.621 + 0.334 × Sex (F = 1, M = 2), R = 0.512, p <0.001. A general linear model, adjusted for gestational age (40-50 days), revealed that mean CRL was significantly higher in male than in female fetuses (4.58-±-0.09-mm, [95% CI: 4.3-4.7] vs 4.24-±-0.09- mm [4.0-4.4]; p < 0.001). Conclusions: Male fetuses are larger than female fetuses in the early first trimester. Given that gonadal differentiation has not yet occurred, still unidentified nonhormonal factors are apparently responsible for this difference.