First trimester fetal heart rate as a predictor of newborn sex

To predict the sex of newborns using first trimester fetal heart rate (FHR).

This was a retrospective review of medical records and ultrasounds performed between 8 and 13 weeks of gestation. Continuous variables were compared using Student's t-tests while categorical variables were compared using Chi-square test.

We found no significant differences between 332 (50.7%) female and 323 (49.3%) male FHRs during the first trimester. The mean FHR for female fetuses was 167.0 ± 9.1 bpm and for male fetuses 167.3 ± 10.1 bpm (p = 0.62). There was no significant difference in crown rump length between female and male fetuses (4.01 ± 1.7 versus 3.98 ± 1.7 cm; p = 0.78) or in gestational age at birth (38.01 ± 2.1 versus 38.08 ± 2.1 weeks; p = 0.67). The males were significantly heavier than females (3305.3 ± 568.3 versus 3127.5 ± 579.8 g; p < 0.0001) but there were no differences in the proportion of small for gestational age (SGA), average for gestational age (AGA) and large for gestational age (LGA) infants.

We found no significant difference between the female and male FHR during the first trimester in contrast to the prevailing lay view of females having a faster FHR. The only statistically significant difference was that males weighed more than female newborns.