Current and future impact of rising multiple birth ratios on low birthweight.

Before 1980, multiple births with their increased risk of low birthweight comprised too small a proportion of births in the United States to have a significant impact on overall low birthweight levels; the recent steep rise in the multiple birth ratio has heightened their influence, however. Between 1980 and 1992 the white multiple birth ratio rose from 18.5 to 24.0 multiple births per 1,000 births. While the overall level of white low birthweight newborns increased slightly between 1980 and 1992 (from 5.7 percent to 5.8 percent), low birthweight for white singletons actually improved, declining from 4.9 to 4.7 percent. Thus, the increase in overall white low birthweight levels is a reflection of the increase in white multiple births and, to a much lesser extent, of the small increase in low birthweight among these births. Black multiple birth ratios and overall low birthweight also rose in this period. The black multiple birth ratio increased from 24.4 in 1980 to 28.2 per 1,000 births in 1992, and low birthweight rose from 12.7 to 13.3 percent. Although low birthweight for black singletons also increased (from 11.5 percent to 11.8 percent), the increase was at a slower pace than for all pluralities combined. Thus, for both white and black births, overall trends in low birthweight mask the disparate patterns of singleton and multiple births. It is, therefore, essential to examine low birthweight trends by plurality to assess accurately changes in this key indicator of infant health.