The relationship between maternal smoking and the incidence of congenital anomalies.

The incidence of congenital anomalies among the offspring of women who never smoked, of those who were past smokers, and of those who smoked during pregnancy was examined for 14,735 children who were members of the Kaiser Foundation Health Plan, a prepaid medical care program in the San Francisco East Bay Area of California. The children were offspring of mothers enrolled in the Child Health and Development Studies, a longitudinal study of pregnancy and the normal and abnormal development of offspring. There were no significant differences in the incidence of congenital anomalies when smokers (all dose levels combined) were compared with those who had never smoked. While there was a slight decrease in the incidence of congenital anomalies among light smokers, a significant difference was observed when the incidence of congenital anomalies among the offspring of women who smoked 20 or more cigarettes daily was compared with that observed among the offspring of women who never smoked. The increase in incidence associated with heavy smoking occurred predominantly among male offspring and was attributed to anomalies classified as moderate. The incidences of two specific anomalies, inguinal hernia and strabismus, were observed to increase significantly when the mothers had smoked during pregnancy. A significant decrease was observed in the incidence of moderate musculo-skeletal anomalies among the male offspring of smokers. The differing nature of the various associations noted stresses the complexity of underlying causal mechanisms.