The relationship between birth weight, smoking during pregnancy and maternal weight gain.

To test the hypothesis that the lower birth weight of infants whose mothers smoke during pregnancy is mediated by lesser weight gain during pregnancy, the authors examined data from an original series consisting of 12,068 births in Northern Finland. Weight gain data were collected for every 10th case chosen by systematic sampling in such a way that every group of abnormal births was treated separately to assure correct representation in the sample. The weight gain of the smokers was 200 g less than that of the controls matched with them for age, parity, place or residence and marital status, but the difference in birth weight and placental weight accounted for about 85% of the total sample in order to quantify the separate and joint contributions to birth weight of smoking, maternal weight gain, pre-pregnant weight, height and duration of gestation. The effect of smoking alone was highly statistically significant in all models. Its effect was independent of weight gain, but when all the variables were introduced simultaneously, the effect was reduced to half. Another set of regression analyses were performed with weight gain as the dependent variable and the others as explanatory variables. The effect of smoking on weight gain was not statistically significant. The result was not affected by including parity in the explanatory variables.