Monthly trend and seasonal variation in twinning rate in Japan, 1975-1994.
We examined birth records from Japanese statistics, 1975-1994, to investigate the seasonality of twin births. We could identify 198,924 pairs of twins (97.9% of all the registered twin records) and estimated the numbers of mono- and dizygotic twin pairs. The seasonal index of the twinning rate for each month was calculated by dividing the crude rate by the estimated trend value for the month. There were significant variations in the seasonal index for overall, dizygotic and monozygotic twinning rates. Peak months with values more than 3% higher than expected were July and October-December for dizygotic twins, and April and June for monozygotic twins; these seasonalities were statistically significant by analysis of variance and the patterns were similar in recent years, with a sharp increase in the total twinning rate. When observed year-by-year, however, there were years that did not show these typical seasonalities. It is suggested that the mechanisms for probable seasonal variations in twinning rates are different for dizygotic and monozygotic twin pregnancies, and that factors involved in these variations are not effective every year.
Nonaka
K
K
Department of Hygiene, Teikyo University School of Medicine, 2-11-1 Kaga, Itabashi-ku, Tokyo 173-8605, Japan. nonaka@med.teikyo-u.ac.jp
Imaizumi
Y
Y
D000328:Adult; D005260:Female; D006801:Humans; D015994:Incidence; D007564:Japan / Q000453:epidemiology; D008875:Middle Aged; D011247:Pregnancy; D012621:Seasons; D014429:Twins, Dizygotic / Q000706:statistics & numerical data*; D014430:Twins, Monozygotic / Q000706:statistics & numerical data*