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In this study the relationship of maternal smoking in pregnancy to low birth weight, fetal growth retardation, preterm delivery, and perinatal deaths has been investigated in a sample of 36,544 women with their single newborns, from 6 Italian centers (Triese, Milan, Parma, Rome, Naples, Bari), where a multicenter survey of perinatal preventive medicine (MPPI) was carried out, between 1973 and 1979, with the financial support of the Consiglio Nazionale dell Ricerche. The proportions of babies weighing 2500 g or less, and of babies with birth weight, head circumference or crown-heel length below the 10th centile were similar among nonsmokers and "stopped" smoker mothers, while they increased with increasing smoking levels. As regards head circumference and crown-heel length below the 10th centile, odds ratios are essentially the same before and after adjustment for age, parity, and socioeconomic class (about 1.5 for "1-10CGT" and up to 2.5 for "11+CGT"). Only slight increases have been observed in the odds ratios concerning low birthweight below the 10th centile (about 1.5 for "1-10CGT" and up to 3.5 for "11+CGT") after adjusting for the socioeconomic demographic variables (i.e. mainly removing the favourable effect of high socioeconomic class or low parity, which more frequently characterize smoker mothers). Owing to the lower smokers' frequencies, cigarette smoking estimated attributable risks concerning low birth weight and "small for dates" babies in the MPPI centres (between 4% and 19%) are considerably lower than those estimated in studies carried out in the US or Canada. Data analyzed in the present study do not indicate any relationship between maternal smoking and preterm delivery or perinatal deaths.