Recent and Long Term Trends of Multiple Birth Rates and Influencing Factors in Japan

Abstract

Multiple birth rates in entire Japan were analyzed using vital statistics for 1951 to 1968 and 1974 to 1992. The twinning rate per 1, 000 births was 6.4 in 1951 and remained fairly constant until 1968, then decreased to 5.8 in 1974 and gradually increased to 7.5 in 1992. The triplet rate was fairly constant for 1951-1974 (56 per million births), then increased with the year up to 1982 (104), and decreased up to 1984 (87), and suddenly increased thereafter (229 in 1992). The average rate of quadruplets per million births for 1951-1968 was 0.93, then increased to 3.3 in 1974 and 6.5 in 1975, and decreased or remained constant until 1984 and suddenly increased thereafter (19.7 in 1992). The rate of quintuplets was 0.86 per million births for 1975-1989 and 2.89 for 1990-1992. The rate of sextuplets was 0.21 per million births for 1975-1992. The higher multiple birth rates since 1975 have been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and since 1985 partially attributed to in-vitro fertilization. J Epidemiol, 1994; 4 : 103-109.