THE EFFECT OF MATERNAL SMOKING ON BIRTH-WEIGHT IN TWIN PREGNANCIES

Information on 869 076 singletons and 17 566 twins, born during the pe riod 1983- 1991, was obtained from the Swedish Medical Birth Registry. Data on birth weight, gestational duration, vital status, and materna l smoking habits during pregnancy were analyzed in order to investigat e whether twinning potentiates the effect of maternal smoking on birth weight and perinatal mortality. The individual birth weights were exp ressed as percentages of mean birth weight, where mean birth weights o f singletons and twins were calculated separately. The birth weight re ducing effect of maternal smoking was found to be of the same magnitud e among twins and singletons weighing > 90% of mean birth weight. For infants weighing < 90% of mean birth weight, maternal smoking had a si gnificantly stronger effect on birth weight among singletons than amon g twins. When gestational duration was taken into consideration, this difference was less pronounced. The effect of maternal smoking on gest ational duration was stronger among singletons than twins. The smoking -related risk increase of perinatal death was of about the same magnit ude among twins and singletons.