Acta geneticae medicae et gemellologiae
-
Records of two hospitals and of an association of mothers of twins were analysed for the years 1924-1980, finding a total of 656,109 and 1185 twin births, respectively. Twinning rates were higher in summer or fall, in general, but these seasonal variations occasionally changed within a short period of years, as in 1973-75 in the Association group. ⋯ The unlike-sexed twinning rate of the mothers born in May-July was also lower than that of the other mothers, and constantly low even at higher maternal ages. A possible explanation for these phenomena is discussed.