Perinatal events associated with maternal smoking during pregnancy.

To identify components of smoking-related increased perinatal mortality, detailed analyses of data from the Ontario Perinatal Mortality Study (50,000 births, 1,300 deaths, 1960-1961) measured the relationship of maternal smoking to birth weight, gestation, placental complications, and perinatal mortality. Cross-tabulations with other factors and multiple adjustment showed increases with amount smoked of birth weights less than 2500 gm, gestations less than 38 weeks, placenta previa, abruptio placentae, and perinatal mortality. These significant, smoking-related increases were independent of mother's height, weight, hospital status, age-parity group, birthplace, previous pregnancy history, weight gain, time of registration, and sex of child. Maternal smoking had the strongest effect on birthweight in the 8 factor regression, and birth less than 2500 gm increased directly with smoking level from 20% to 340% in 37 data subgroups. Births less than 38 weeks increased 20% and 50% and perinatal mortality increased 20% and 35% for less than 1 pack and 1 + pack smokers, respectively, adjusted for 7 other factors. Placental complications increased consistently with smoking level in all of 37 subgroups except for primiparous less than 1 pack smokers. Adjusted rates increased 25% and 92% for placenta previa, 23% and 86% for abruptions among smokers of less than 1 pack and 1 + packs, respectively. These complications carry high perinatal mortality risk, and account for one-third to one-half of the perinatal deaths attributable to maternal smoking.