Trends in birthweight in the north of England.

This article describes trends in population birthweight distribution in the north of England between 1982 and 2000, and reviews the international literature on recent birthweight trends in industrialized populations. Two contrasting trends were observed: an increase in low birthweight from 7.0% to 7.7% of all births, and an increase in births weighing 3500 g or more, consistent with an increase in mean birthweight. The rate of multiple births increased over the study period, and multiple births accounted for about half of the increase in low birthweight. Multiple births are rising due to increasing maternal age, and the development and use of assisted reproductive technologies. Low birthweight increased in both multiple and singleton births, due entirely to increased low-birthweight live births. There was a substantial reduction in low-birthweight stillbirths. The rise in low-birthweight live births is multifactorial, but changes in registration practice and increased elective delivery are likely contributory factors. The increase in high birthweights is likely to reflect reductions in smoking during pregnancy, and increased maternal height and weight. The dramatic recent rise in maternal body mass index (BMI) at booking is likely to have a substantial impact on birthweight and adverse perinatal outcomes in coming decades.