Interaction of smoking and maternal weight status in influencing infant size.

: The effects of pregravid weight, gestational weight gain and smoking on infant size, and the extent to which weight gain can compensate for smoking behaviour are explored in multivariate analysis of 1,330 healthy mothers and their full-term infants at the PEI Prenatal Nutritional Counselling Program. Stratified analysis by pregravid weight and smoking status indicated that gestational weight gain was linearly related to birthweight. Gestational weight gain significantly predicted the risk of small-for-gestational-age infants (SGA) only among smokers. Increased weight gain partially mitigated the negative effect of smoking on birthweight but among underweight smokers there was a high risk of SGA (> 10%) despite an elevated gestational weight gain. Level of smoking was directly related to infant birthweight, with substantial reduction even for light smokers. Complete smoking cessation is important for preventing low birthweight in Canada, particularly among underweight mothers.