Impact of multiple births and elective deliveries on the trends in low birth weight in Norway, 1967-1995.

To describe trends in low birth weight (less than 2,500 g), the authors analyzed 1.7 million live births and stillbirths registered between 1967 and 1995 in the Medical Birth Registry of Norway. The proportion of low birth weight infants declined from 5.3% in 1967 to 4.5% in 1979 and was followed by a steady increase that reached 5.3% in 1995. Similar trends were observed in the proportion of preterm births. Mean birth weight increased from 3,456 g in 1967 to 3,518 g in 1995. From 1979 to 1987, the increase in the prevalence of low birth weight was related to single births, and after 1987 it was related to multiple births, which increased from 2.3% of all births in 1987 to 3.1% in 1995. The proportion of low birth weight in births occurring after 37 weeks of gestation declined continuously, resulting in low birth weight births' to an increasing extent being made up of births occurring before 37 weeks of gestation. In an ecologic analysis based on county of maternal residence, the increase in low birth weight among single births was accounted for by an increase in deliveries with induction of labor or cesarean section. The authors conclude that the overall proportion of low birth weight births is not a good indicator of health in a population with extensive use of obstetric procedures that affect gestational age or assisted fertilization, which increases the number of multiple births.