Presented are the results of multivariate regression analysis of the variations in birth weight of term babies associated with the socioeconomic, physical, and medical factors recorded in obstetric case notes at a London teaching hospital. The results show that the sex of the baby, parity, maternal smoking during the pregnancy, maternal height, weight, marital status, and race, and gestation (after allowing for the foregoing characteristics) were all important and significant factors. Socioeconomic status, per se, was not, nor was age. In particular, the results negate the suggestion that the loss of birth weight associated with smoking during the pregnancy might be due to the smoker, rather than to smoking itself. The results indicate that moderate smoking (one to 15 cigarettes per day) causes a loss of 107 gm, and that heavy smoking (16+) causes a loss of 158 gm, after all other factors have been taken into account. An investigation of how these figures would be affected by failing to control for one or more of the other factors led to the conclusion that the estimates of the loss of birth weight as a result of smoking in completely uncontrolled studies may be reasonably accurate, whereas the estimates in partially controlled studies are probably biased.
PIP: A multivariate regression analysis was made of the variations in birth weight of term infants associated with socioeconomic, physical, and medical factors. The data were taken from the case notes of a series of 1115 obstetric patients who had given birth at the West London Hospital, Hammersmith, London. The sample included all the births recorded at the hospital during 5 separate months drawn from the October 1977 to August 1979 period. The sex of the infant, parity, maternal smoking during pregnancy, maternal height, weight, marital status, race, and gestations (after allowing for the foregoing characteristics) were found to be important and significant factors. Of these characteristics, smoking during pregnancy was the most contentious factor. Moderate smokers (15 or fewer cigarettes a day) had infants 197 g lighter than those of nonsmokers. Heavy smokers (16 or more cigarettes) had infants 158 g lighter, after all other factors had been taken into account. Female infants weighed 118 g less than male infants. There was a very strong effect of height on birth weight, even when all other factors were controlled. Infants born to mothers in the lower quartile were 132 g light. The heavier the mother at booking, the heavier the child. Asian infants were 100 g lighter and black Africans 327 g lighter than the reference group, those with mothers born in the UK. The non-Caucasian estimate in the fully specified regression, 122 g light, was highly significant. Socioeconomic status, per se, and age were not significant factors associated with birth weight. When gestation was treated as a continuous variable, it had a coefficient of 30.6 g/week, significant at the 0.1% level. It had a negative coefficient significant at the 0.1% level when a quadratic term was introduced, indicating a progressive decline in the increase in birth weight per week of pregnancy.