Maternal Blood Pressure Before Pregnancy and Sex of the Baby: A Prospective Preconception Cohort Study.

BACKGROUND Population-level sociologic studies have suggested that adverse societal conditions may affect fetal viability in a sex-specific manner and thereby modify the ratio of male vs. female babies. This concept suggests that there may exist certain physiologic features in a woman that relate to her likelihood of delivering a boy or girl. We thus established a preconception cohort to prospectively evaluate the relationship between maternal pregravid health and sex of the baby. METHODS In this analysis nested within an observational cohort study, 1,411 newly married women in Liuyang, China, underwent pregravid cardiometabolic characterization (including anthropometry and measurement of blood pressure, cholesterol, triglycerides, and glucose) at median 26.3 weeks before a singleton pregnancy, delivering at 39.0 ± 1.3 weeks gestation. RESULTS Systolic blood pressure before pregnancy was higher in women who delivered a boy than in those who had a girl (112.5 ± 11.9 vs. 109.6 ± 12.0 mm Hg, P < 0.0001). The prevalence of a male baby progressively increased across quintiles of pregravid systolic blood pressure (P < 0.0001). After covariate adjustment, mean adjusted pregravid systolic blood pressure was higher in mothers of boys vs. girls (106.0 vs. 103.3 mm Hg, P = 0.0015). On logistic regression analysis, pregravid systolic blood pressure emerged as the only significant predictor of having a male baby (adjusted odds ratio = 1.017 per mm Hg, 95% confidence interval = 1.007-1.028). The pregravid difference in blood pressure between mothers of boys and girls was not present during any trimester of pregnancy. CONCLUSION Maternal blood pressure before pregnancy is a previously unrecognized factor that may be associated with the likelihood of delivering a boy or girl.