Interpregnancy weight gain and the male-to-female sex ratio of the second pregnancy: a population-based cohort study

ObjectivesTo investigate whether interpregnancy maternal weight change (difference between body mass index [BMI] at the first antenatal visit of the second pregnancy and BMI at the first antenatal visit of the first pregnancy) or changes in smoking status between pregnancies is related to the sex ratio of the second pregnancy.DesignPopulation-based cohort study.SettingSwedish Birth Registry.Participant(s)A total of 220,889 women who had their first two consecutive singleton births between 1992 and 2004. Both live births and stillbirths were included.Intervention(s)Analyses of data collected prospectively in nationwide registries.Main Outcome Measure(s)Male-to-female sex ratio of the second pregnancy.Result(s)The sex ratio of the second pregnancy increased linearly with the amount of maternal weight change from the first to the second pregnancies, from 1.024 in women who lost more than 1 unit BMI to 1.080 in women who gained 3 or more units. This association was independent of obstetric complications, length of the interpregnancy interval, and maternal sociodemographic characteristics at the second pregnancy. Change in maternal smoking status from the first to the second pregnancies was not significantly related to the sex ratio of the second pregnancy.Conclusion(s)There could be a causal relation between prepregnant maternal weight gain and the sex ratio of the offspring.