Twinning rate by month of mother's birth in Japan.
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 twinning rate was also affected by the month of mother's birth. It was low among the mothers born around May-July in maternities up to 1960, and these mothers did not show seasonal variation of twin births. 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.
D000328:Adult; D001723:Birth Rate; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D007564:Japan; D008297:Male; D008423:Maternal Age; D011247:Pregnancy; D012621:Seasons; D012737:Sex Factors; D014427:Twins