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.