Female versus Male Fetuses—Variation in Fetal Heart Rate Tracing and Adverse Outcomes at Term

Objective: To compare the fetal heart rate tracing (FHRT) and composite adverse outcomes among female versus male newborns delivered at term. Study Design: The inclusion criteria of the retrospective study were non-anomalous singletons, delivered at > 37 weeks after labor, whose sex assignment at birth was either female or male. The consecutive deliveries occurred over 15 months. Obstetricians—blinded to maternal characteristics and outcomes—interpreted the FHRT for the last 20 to 120 min of labor. Composite neonatal and maternal adverse outcomes (CNAO and CMAO) were compared between the groups. Adjusted odds ratio (aOR) and 95% confidence intervals (CI) were calculated. Results: Of the 5,160 deliveries during the study period, 3,165 (61.3%) met the inclusion criteria and among them 1,514 (47.8%) were females and 1,651 newborns (52.2%), male. Obstetricians reviewed 358,620 minutes of FHRT. Compared to female, the FHRT among male was significantly more likely to have severe late decelerations (aOR 5.25; 95% CI 1.16-23.83), or any severe decelerations (aOR 1.21; 95% CI 1.03 – 1.42). Whether the tracing were categories I, II or III were similar. Cesarean delivery for non-reassuring FHRT was more common among male than female newborns (aOR 1.21; 95% 1.03-1.41). CNAO for male was significantly higher than female newborns (aOR 2.55; 95% CI 1.35-4.84). Apgar score < 7 at 5 min was the component of the CNAO that differed significantly (aOR 3.16; 95% CI 1.27-7.86). CMAO was also significantly higher for individuals who delivered male (9.3%) vs female (6.3%) newborns (aOR 1.51, 95% CI 1.16-1.98). Estimated blood loss > 1,000 mL was significantly more likely among male than female newborns (aOR 1.51; 95% CI 1.09-2.08). Conclusion: Among singleton pregnancies delivered at term, male, compared to female, fetuses are significantly more likely to have abnormalities of fetal heart rate tracings, and adverse outcomes.