Development and sexual difference in embryonic heart rates in pregnancies resulting from in vitro fertilization.

The purpose of this study was to investigate the gestational change of embryonic heart rates (EHRs) and to estimate the influence of embryonic sex on the EHR in pregnancies resulting from in vitro fertilization in the early first trimester. With transvaginal ultrasonography, we performed 92 and 105 examinations, on 27 male and 30 female embryos, respectively. The EHR increased gradually from 87 beats per min at 38 days of gestation to 189 beats per min at 62 days of gestation. The relationship between gestational age and EHR was explored by regression analysis: male EHR (beats per min) = 3.78 x gestational age (days) - 51.30 (r = 0.95), female EHR = 3.65 x gestational age - 44.56 (r = 0.95). These regression lines were mutually included in the 95% confidence intervals for each other. Our results indicate both a close positive correlation between the EHR and gestational age and no statistically significant difference in the EHR between male and female embryos. These findings suggest that the EHR measurement is a novel method for very early ultrasound dating with the identical EHR criterion without regard to embryonic sex.