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epidemic factors

Acta geneticae medicae et gemellologiae: twin research (1)

International Soc for Twin Studies (1)

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Alternate Changes in Birth Seasonality of Twins During 1971–1984 in Japan
I. Nakamura, Y. Amau, K. Nonaka, T. Miura
Journal:

Acta geneticae medicae et gemellologiae: twin research / Volume 36 / Issue 3 / July 1987

Published online by Cambridge University Press:

01 August 2014, pp. 373-380

Print publication:

July 1987
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Birth dates of 1,536 twin-pairs in 1971-1984 were collected from the members of an association of twins' mothers. The seasonal variation of twinning changed every 2-4 years. Years when twinning rate was higher in the summer-fall season (1971-72, 1976-77, 1982-84) and those when a peak of the rate was not observed (1973-75, 1978-81) appeared alternately. In years with a summer-fall peak, the elevation of twinning in the summer-fall season was detected consistently in both like- and unlike-sexed and in both MZ and DZ twin groups. The twinning seasonality in these years, however, was not evident in twin births of mothers who were born in May-July. These results suggest the possibility that seasonal factors which influence the twinning rate be not multiple-ovulation-inducing but probably abortion-inducing factors and most likely seasonally epidemic microbes.