MATERNAL SMOKING, BIRTH-WEIGHT AND GESTATIONAL-AGE IN SUDDEN-INFANT-DEATH-SYNDROME (SIDS) BABIES AND THEIR SURVIVING SIBLINGS

To evaluate the effect of maternal smoking on intrauterine growth of b abies who died of sudden infant death syndrome (SIDS), birthweights of SIDS infants and their surviving siblings were compared with birthwei ghts of infants in sibships were all infants survived the first year o f life. We studied 184 349 mothers with at least two births registered in the population-based Swedish Medical Birth Registry during 1983-91 . The mother being the unit of analysis, birthweight and gestational a ge of her infants were the repeated measures used in a repeated measur es analysis of variance. Mothers whose first two infants survived at l east 1 year, smoked less than mothers of SIDS infants, 25 and 41% (P < 0.01). Overall, SIDS mothers did not smoke more while pregnant with t he SIDS infant than while pregnant with the surviving sibling. SIDS si blings weighted, on average, 90 g less than infants in non-affected si bships. SIDS babies were even lighter, 193 g, and had 3.8 days shorter mean gestational age, compared with same birth-order babies in nonaff ected sibships. After adjustment for gestational age, the birthweight difference changed only slightly for SIDS siblings, while the differen ce for SIDS infants was reduced from 193 to 110 g. Further adjustment for smoking reduced the birthweight difference for SIDS siblings, from 74 to 50 g, and SIDS infants, from 110 to 82 g. Intrauterine growth r etardation of sibships with a SIDS baby is explained only partly by ma ternal smoking. The even lower birthweight of the SlDS baby, resulting from shorter gestational age, cannot be explained by smoking, suggest ing pregnancy factors specific to the SIDS baby and not to its sibling s.