Birth Weight

A definite, well-established relationship exists between smoking and low birth weight defined as birth weight less than 2500 grams. In general, the average reduction in birth weight seen in smoking women is 200 grams. This results in a doubling of the incidence of low birth weight infants. The risk and magnitude of low birth weight is related to the number of cigarettes smoked during pregnancy. The critical period during which smoking exerts its deleterious influence has not been determined, but it has been shown that if women cease smoking during pregnancy the infant’s birth weight will be comparable to a non-smoker.

The exact mechanism for decreased birth weight is unclear. At the present, there is evidence that smokers do not consume fewer calories or have less weight gain during pregnancy so it has been concluded that the decrease in birth weight seen in infants of smokers is not due to nutritional factors. Anthropomet- ric studies comparing the differences in body composition in infants of women who smoke with those of non-smokers have found a decrease in the fat-free mass. Specifically, weight and length were decreased in the infants of smokers, but no differences in skinfold and limb circumference measurements were seen. It is unknown whether the physiological effects of smoking on fetal growth are due to the vasoconstrictive properties of nicotine on uterine blood or the decreased oxygen availablity due to carbon monoxide and the formation of carboxyhemoglobin.