Objectives: To describe changes in rates of higher-order multiple births (triplets and higher) between 1972 and 1989, to compare infant mortality rates in infants of higher-order multiple births and singletons born from 1983 through 1985, and to compare infant mortality rates among higher-order multiples born from 1983 through 1985 with rates among those born in 1960.
Research design: Population-based analysis of live births (1972 through 1989) and infant deaths (1960 and 1983 through 1985) in the United States. The rate of higher-order multiple births was calculated per 100,000 live births.
Data source: Computerized national natality files for 1972 through 1989 and national linked birth/infant death data sets for 1960 and 1983 through 1985 from the National Center for Health Statistics, Centers for Disease Control.
Population: Live births to white and black women in the United States.
Main results: Between 1972 through 1974 and 1985 through 1989 the rate of higher-order multiple births increased by 113% among infants of white mothers and by 22% among infants of black mothers. In whites the increase was mostly age specific and was not due to the upward shift in the maternal age distribution. The increase was particularly large in white women aged 30 through 34 years (152%) and 35 through 39 years (165%) and in more highly educated mothers. In blacks the modest increase in the rate of higher-order multiple births was mostly due to an upward shift in the maternal age distribution. From 1983 through 1985, mortality of infants of higher-order multiple births was about 15 times that of singletons. This was due almost entirely to the lower birth weight distribution of infants of higher-order multiple births. Their weight-specific mortality compared favorably with that of singletons. At 500 through 999 g, mortality was about the same. In weight categories between 1000 and 1999 g, mortality rates in higher-order multiple births were much lower: weight-specific relative risks ranged from 0.30 to 0.73. Between 1960 and 1983 through 1985 infant mortality in higher-order multiple births declined by about 50%.
Conclusions: It is likely that much of the increase in the incidence of higher-order multiple births is due to the rise in the use of ovulation-inducing drugs for the treatment of infertility. This increase and the decline in mortality risk have created a much greater need for medical and social services for infants of higher-order multiple births and their families.