Analysis of multiple birth rates in Japan
Over-all triplet birth rate in Japan was decreased during 1955 to 1967 and 1974. Monozygotic rate was slightly decreased with the year, but dizygotic rate as estimated from monozygotic and dizygotic twinning rates was decreased with the year. Trizygotic rate was increased with the year and particularly high in 1974.
Monozygotic rate was slightly increased up to the maternal age group of 35–39 years and decreased thereafter. Similar but more marked pattern is seen for dizygotic rate, whereas trizygotic rate seems to be independent of maternal age. Analysis of secular change during the period from 1960 to 1967 indicated a clear decrease of monozygotic rate in the older maternal age groups. The maternal age effect on the increase of trizygotic rate is less clear.
The highest monozygotic rate was seen in the northeast Hokkaido District (56 per million). On the other hand, dizygotic rate was slightly higher in the northeast than in the southwest. As to trizygotic rate, the highest rate was seen in the southwest Kyushu District (10 per million). Heterogeneity of geographical variation was indicated in monozygotic and trizygotic rates, but not in dizygotic rate.
Underrepresentation of unlike-sexed triplet sets was suggested in two districts. After excluding the two districts, corrected monozygotic rate is 30, and dizygotic and trizygotic rates are 18 and 7 per million, respectively.
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