[Birth weight of infants born without congenital defects. II. Effect of tobacco and parity of the mother on the wight of the newborn infant].

We analyzed the effect that number of gestations, smoking and number of cigarettes per day during pregnancy have on the birthweight using a sample of 14,097 liveborns without birth defects identifiable within the first three days of life. To our purposes, we controlled the effect of gestational age, sex and maternal age through multiple regression analysis. From the results we concluded that the birthweight is increased 39.80 g per each pregnancy of mothers, showing a progressive but smoothed increase by a negative quadratic effect. Smoking in mothers decreases the weight of newborns and has no effect on the gestational age. The decrease of birthweight was stronger when we looked at the number of cigarettes per day, showing a diminishing trend when the number of cigarettes per day, showing a diminishing trend when the number of cigarettes is increased, but again smoothed by a positive quadratic effect. From the multiple regression analysis controlling some factors we got a formula that permitted to us to estimate the average in decreasing weight depending on the number of cigarettes that mother smokes during pregnancy.