Maternal cigarette smoking during pregnancy and infant ponderal index at birth in the Swedish Medical Birth Register, 1991-1992

Maternal smoking during pregnancy reduces mean birthweight(1-4) and increas es the risk of small-for-gestational-age infants.(4-8) The ponderal index i s a measure of birthweight in relation to crown-heel length and is used to classify "symmetric" growth retardation (with normal ponderal index) and "a symmetric" growth retardation (with low ponderal index). Asymmetric infants are thinner and have less birthweight per centimeter of length than symmet rical infants. Intrauterine growth-retarded infants with symmetric body pro portions have higher rates of neonatal complications(9,10) and higher rates of hospitalization in the first year of life than asymmetric infants,(11) and they lag behind asymmetric infants in later physical growth.(12-15) Earlier studies of the effect of maternal smoking on infant ponderal index reported no statistically significant effect,(1,2,16,17) but one investigat ion reported a reduction in ponderal index among the infants of smokers.(18 ) Recognizing that ponderal index is strongly correlated with the degree of birthweight reduction,(19) the present study is the first to control for b oth gestational age and birthweight.