Analysis of multiple birth rates in Japan. VI. Quadruplets: birth and stillbirth rates.

Abstract

Overall quadruplet birth rates per million deliveries were 1.14 and 3.29 during the period from 1955 to 1967 and in 1974, respectively. The higher quadruplet birth rate in 1974 might be attributed to higher proportion of mothers treated with ovulation-inducing hormones. Estimated rates of monozygotic, dizygotic, trizygotic and tetrazygotic quadruplet deliveries per million were 0.78, 0.19, 0.04, and 0.23, respectively, during the period from 1955 to 1967 and in 1974. The monozygotic quadruplet rate was 2.7 times higher in Japan than in Europe and USA, the dizygotic and tetrazygotic quadruplet rates of the former were a half of the latter, and the trizygotic quadruplet rate of the former was one tenth of the latter. Stillbirth rates of quadruplets were 0.882 during the period of 1951–1959, 0.650 of 1960–1968 and 0.607 in 1974, a two-third decrease in around 20 years. Effects of maternal age and birth order on stillbirth rate of quadruplets were not indicated because of a small sample size. Effects of gestational age and weight at birth on stillbirth rate were also examined.