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The Twinning Rates by Zygosity in Japan, 1975-1994

Published online by Cambridge University Press:  01 August 2014

Y. Imaizumi  and
K. Nonaka
Y. Imaizumi*
Affiliation:
National Institute of Population and Social Security Research, Ministry of Health and Welfare, Kasumigaseki, Chiyoda-ku, Tokyo, Japan
K. Nonaka
Affiliation:
Department of Hygiene, Teikyo University, School of Medicine, Kaga, Itabasiku, Tokyo, Japan
*
National Institute of Population and Social Security Research, 1-23, Kasumigaseki, Chiyoda-ku, Tokyo 100, Japan

Abstract

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The monozygotic (MZ) twinning rate in Japan had remained nearly constant from 1975 (3.74 per 1,000 births) to 1994 (4.23), whereas the dizygotic (DZ) twinning rate had remained nearly constant from 1975 (1.86) to 1986 (2.27), and had gradually increased up to 1994 (3.89). The higher DZ twinning rate since 1987 has been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to in-vitro fertilisation in Japan. As for maternal age, MZ twinning rates have remained nearly constant for maternal age groups except the youngest and the oldest age groups. On the other hand, DZ twinning rates increased up to the 35-39 years of age group and decrease thereafter. In 1994, the twinning rate was higher in DZ twins than in MZ twins for maternal age groups of 30-34 years and of 35-39 years. As for geographical variations in twinning rates, DZ rates statistically significantly increased with the year during the period from 1986 to 1994 in 31 out of 47 prefectures. In 1994, twinning rates in 21 out of 47 prefectures were higher in the DZ than the MZ rate, and the DZ rate is equal to the MZ rate in two prefectures. Geographical variations in twinning rates by zygosity in 1994 drastically changed from those during the period from 1955-1959 and in 1974.

Keywords

Twinning ratesZygosityMaternal ageGeographical variations
Type
Research Article
Information
Acta geneticae medicae et gemellologiae: twin research , Volume 46 , Issue 1 , January 1997 , pp. 9 - 22
Copyright
Copyright © The International Society for Twin Studies 1997

References

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