From a population of singleton pregnancies, 152 overweight women (greater than 110% of standard) were matched with normal weight women (95-110%) for age, height, parity, race, and smoking habits. Comparisons were made of initial weight (weight at the first prenatal visit) and gestational weight gain and pregnancy outcome. Mean birth weights and gestational ages of infants of normal and overweight women were not significantly different. For normal weight women birth weight increased significantly as height, initial weight, and body mas index increased (p less than 0.01), but no such relationship existed for overweight women. The lack of effect of initial weight on birth weight in overweight women is attributable, in part, to the significantly less gestational weight gains of these mothers (6.3 kg vs 8.2 kg). When normal and overweight gravida had gestational weight gains of less than 7 kg, offspring of overweight mothers were significantly heavier. Gestational weight gain was positively correlated with birth weight for both normal (p less than 0.0001) and overweight women (p less than 0.001). Within the overweight and normal weight groups, smokers had lower initial weights and gestational weight gains than nonsmokers. Offspring of normal weight smokers had a mean birth weight 232 g less than that of nonsmokers (p less than 0.01). The difference in birth weight between overweight smokers and nonsmokers (135 g) was not statistically significant. While there is substantial data to support a weight gain of 10-12 kg in normal weight gravida, it would appear that a gain of approximately 7 kg in overweight middle class women does not impair fetal growth as measured by birth weight.(ABSTRACT TRUNCATED AT 250 WORDS)