Twinning Rate by Month of Mother's Birth in Japan

Abstract

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.

Alternate Changes in Birth Seasonality of Twins During 1971–1984 in Japan

“…In the present study, in agreement with the previous report [14], the mothers born in May-July did not show a peak of twinning in the summer-fall season, while the other mothers showed an excess in that season. To explain this phenomenon, we can also hypothesize immunogenic effeets of the above-mentioned seasonally epidemie factors.…”

“…This discrepancy from Weinberg's rule may not be explained until the zygosity of our UZ-LS twins is determined. The present analysis confirms the excess of twin births in the summer-fall season and its occasionai loss in agreement with our previous report [14]. It also shows that the years showing a summer-fall peak and the years showing no distinct seasonality appeared alternately in recent years.…”

“…The cause of seasonality of DZ twinning and that of total twinning in Europe, North America and Africa, where DZ twinning is predominant, might be explained by seasonally induced multiple ovulations. In Japan, where the MZ twinning rate is higer than the DZ twinning rate, however, the seasonality was detected not only in total twinning [10,14] but also in both MZ and DZ twinning separately [7].…”

“…There have been few secular analyses of seasonality of twinning [5,15]. We analysed twin births for the years 1924-1980 among records of two maternity hospitals and of an association of mothers of twins and revealed that twinning rates were higher, in general, in summer or fall, but this seasonality occasionally disappeared within a few years, as was observed in 1973-1975 [14]. Seasonal excess or reduction of twinning may not be consistent during a long period of time.…”

Conditions at conception in women with recurrent miscarriage

“…To our knowledge, this study is the first in which an association is found between month of birth and habitual miscarriage. Associations with month-of-birth have been described, though, for a great variety of features, among them several reproductive features such as early menarche (Jongbloet et al, 1994), menstrual disorders (Jongbloet et al, 1994), mean menstrual cycle length (Nakamura et al, 1987), twin proneness (Miura et al, 1984;Nonaka et al, 1993), and early and late menopause (Jongbloet et al, 1994).…”