Gender-related differences in fetal heart rate during first trimester

Many expecting parents wish to ascertain fetal gender early in pregnancy. Our goal was to determine whether fetal heart rate (FHR) of males and females during the first trimester is significantly different.

From November 1997 to February 2003 we enrolled pregnant women with singleton gestations who underwent obstetric sonography at less than 14 weeks of gestational age. Indications for the sonographic study included first-trimester bleeding, uncertain gestational dating, poor obstetrical history, and aneuploidy screening by nuchal translucency. The sonographic studies were performed by a single sonographer and reviewed by the first author. The FHR was determined by m-mode. All subjects underwent second-trimester sonography at 18.0-24.0 weeks' gestation by the same team, and fetal gender was recorded. Multiple gestations, miscarriages and pregnancies with uncertain fetal gender were excluded. Sonographically assigned fetal gender was confirmed at delivery.

Of the 966 first-trimester studies performed, 477 met the inclusion criteria. Of these, 244 (51%) were female and 233 (49%) were males. There were no statistical differences in mean maternal age, gravidity, parity, and mean gestational age at the time of the first study (9.0 +/- 2.3 weeks for female fetuses and 9.0 +/- 2.3 weeks for males, p = 0.7). The average female FHR was 151.7 +/- 22.7 bpm and male FHR was154.9 +/- 22.8 bpm (p = 0.13).

Contrary to beliefs commonly held by many pregnant women and their families, there are no significant differences between male and female FHR during the first trimester.