Trends in multiple births.

Since the early 1970s, the number and rate of multiple births have increased fairly steadily. Of the 4,110,907 babies born in the United States in 1991, 98,125 (2.4 percent) were twins, triplets or other higher order plural births. The 1991 rate of 23.9 multiple births per 1,000 live births--the highest recorded in this country in the last 50 years--was 1.3 percent higher than that in 1990, and 14 percent higher than the rate in 1985. The twins proportion of all multiple births has been slowly decreasing--97.6 percent in 1985, 96.9 percent in 1990 and 96.6 percent in 1991. Rates of multiple births continue to be higher among black women than white (2.8 and 2.3 per 1,000 live births, respectively, in 1991) and are more prevalent among older women. In 1991 the rates were higher through each age group to a high of 3.4 for white and 3.6 for black mothers aged 35-39 before dropping to 1.9 and 0.5, respectively, among 45- to 49-year-olds. Incidence of low birthweight (< 2,500 grams) and of very low birthweight (< 1,500 grams) babies were more frequent among multiple births than singletons and were more common among black than white infants. The median weight for white singleton births was 3,420 grams versus 2,260 grams for black plural births. The District of Columbia, Michigan and Massachusetts registered the highest percentage of multiple births in 1991 (2.9 to 2.7 percent) and the lowest rates were recorded in New Mexico and Wyoming (1.9 percent).