Interplay between maternal weight and seasons in determining the secondary sex ratio of human offspring.

When gender of 9,284 single fetuses, which were delivered at term in the Policlinic of Modena between 1997-2001, was stratified for month of conception and for quartiles of pregravid maternal weight, it showed that the ratio of male to female fetuses (secondary sex ratio) was characterized by a clear seasonal variation that was modulated by pregravid maternal weight. Two seasonal peaks of sex ratio (March and October +/- 31 days) were observed in mothers in the lowest two quartiles of prepregnancy body weight (< or =62 kg), and one single peak (October +/-36 days) was observed in mothers with preconception weight in the upper two quartiles.