BACKGROUND: Previous studies suggest that RhD positive heterozygotes express better health status than RhD positive homozygotes and especially RhD negative subjects. This also applies to pregnant women. According to the Trivers-Willard hypothesis, women in better physical condition should have more sons. AIM: To test the hypothesis that RhD positive heterozygous mothers have a male-skewed sex ratio. STUDY DESIGN: Cross-sectional study. The data was analysed using Chi-Square test for all women, separately for RhD positive and RhD negative women, and separately for primiparous and multiparous women. The effects of maternal weight as a continuous predictor and the RhD phenotype of newborn as a categorical predictor of newborn sex were evaluated by the generalized linear model (GLZ) separately for RhD positive and RhD negative women using binomial distribution and logit link function. OUTCOME MEASURES: Clinical records comprised maternal weight before pregnancy, number of previous deliveries, sex of the newborn, maternal RhD phenotype, and RhD phenotype of the newborn. SUBJECTS: We analysed data from 5655 women who gave birth between 2008 and 2012 in General University Hospital in Prague. RESULTS: Secondary sex ratio was significantly higher (P = 0.028) in RhD positive mothers who had RhD negative newborns, i.e., in heterozygotes (SR = 1.23), than in RhD positive mothers who had RhD positive newborns, i.e., in a mixed population of heterozygotes and homozygotes (SR = 1.00), especially in primiparous women (P = 0.013; SR = 1.37 and 0.99 resp.). CONCLUSION: The sex ratio at birth was significantly higher in RhD positive mothers who had RhD negative newborns than in RhD positive mothers who had RhD positive newborns.

Department of Obstetrics and Gynaecology General University Hospital and 1st Faculty of Medicine Charles University Prague CZ 128 08 Prague 2 Czech Republic

Department of Philosophy and History of Science Faculty of Science Charles University Prague CZ 128 44 Prague 2 Czech Republic

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Faster life history strategy manifests itself by lower age at menarche, higher sexual desire, and earlier reproduction in people with worse health