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.

Serum gonadotropin, estradiol, prolactin and alpha-1-antitrypsin levels were measured in the first four days of the menstrual cycle in 14 women who were cycling normally. FSH, and to a lesser extent LH levels, were significantly higher in a group of 8 women who had at least one set of DZ twins (6 of whom had 2 sets) than in a control group of 6 women with no DZ twins. Estradiol levels also tended to be higher in mothers of twins but there were no significant differences in prolactin or alpha-1-antitrypsin concentrations. There were no differences between the means of the two groups in age, height, weight or number of pregnancies.

Mothers of twins, as compared with mothers of singletons, are taller, and for a given height are heavier; they have slightly earlier menarche and menopause. Our recent studies have shown that mothers of DZ twins are at greater risk of cancer of the pancreas (but not for cancers of other sites), as well as of death due to diabetes, other endocrine diseases, and allergies. These findings are consistent with the clinical and epidemiological literature. We have also found that singleton siblings of twins are slightly heavier at birth than children born into sibships consisting of singletons only. The need for further research on the characteristics and long-term health of mothers of twins is indicated.