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The twinning rates by zygosity in Japan, 1975-1994
1National Institute of Population and Social Security Research, Tokyo, Japan.
Abstract:
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.
Insights
The dizygotic (DZ) twinning rate in Japan significantly increased from 1987, linked to fertility treatments. This trend reversed historical patterns, with DZ twins surpassing monozygotic (MZ) twins in frequency by 1994.
Area of Science:
- Reproductive Epidemiology
- Human Genetics
- Demography
Background:
- Monozygotic (MZ) and dizygotic (DZ) twinning rates exhibit distinct demographic trends.
- Historical twinning rates in Japan showed relative stability for MZ twins and a slower rise for DZ twins until the late 1980s.
Purpose of the Study:
- To analyze trends in MZ and DZ twinning rates in Japan from 1975 to 1994.
- To investigate the influence of maternal age and geographical variations on twinning rates by zygosity.
Main Methods:
- Analysis of national birth data from 1975 to 1994.
- Statistical comparison of MZ and DZ twinning rates across different maternal age groups.
- Examination of geographical variations in twinning rates by zygosity across Japanese prefectures.
Main Results:
- The DZ twinning rate increased significantly from 1987 onwards, while the MZ twinning rate remained relatively stable.
- Assisted reproductive technologies, including ovulation-inducing hormones and in-vitro fertilization, are identified as contributing factors to the rising DZ rate.
- DZ twinning rates showed a notable increase with maternal age up to 35-39 years and varied significantly across Japanese prefectures by 1994, diverging from earlier patterns.
Conclusions:
- The rise in DZ twinning rates in Japan since the late 1980s is primarily associated with increased use of fertility treatments.
- Maternal age and geographical location are significant factors influencing twinning rates, with DZ twinning becoming more prevalent than MZ twinning in specific age groups and regions by 1994.