Smoking during pregnancy. Its association with other maternal factors and birthweight.

Data on 293 small-for-dates (SFD), 274 average-for-dates (AFD) and 338 large-for-dates (LFD) pregnancies are analyzed. 60.7% mothers in the SFD group smoked compared with AFD 29.2% and LFD 15.7%. Mean maternal height, weight and weight gain was lowest in the SFD group and highest in the LFD group. The mean weight-for-height index was the same in the SFD and AFD groups and higher in the LFD group. Within each group no differences were found between smokers and non-smokers for height, weight and weight-for-height indices. In the AFD group net pregnancy weight gain was higher for smokers than non-smokers. Feto-placental ratios were similar in all three groups and no differences were found between smokers and non-smokers. More infarcted placentas were found in the SFD group, but the proportions for smokers and non-smokers were alike. The incidence of bleeding during pregnancy and pre-eclampsia was the same for all groups, and between smokers and non-smokers. Hypertension during pregnancy occurred more often in the SFD group, and was more prevalent amongst non-smokers than smokers. The reduction in birthweight of infants born to smoking mothers appear to be a direct effect.