This study is designed to examine the impact of maternal body mass index (BMI), proportional weight gain, and other variables on fetal growth. From a 10% random sample of 5722 Norwegian and Swedish para one and two women (n = 561), three hundred and sixty-nine women for whom prepregnant weight and height were recorded and for whom four fetal ultrasound measurements were taken at 17, 25, 33, and 37 weeks of gestation, were divided into low, average, and high body mass index groups (weight/height2). Fetal growth rate (mm/day) was determined by taking the mean of three measurements of the sagital and transverse diameters of the fetal abdomen (MAD) in each of three study time periods: weeks 17 to 25, first period; weeks 25 to 33, second period; weeks 33 to 37, third period. Proportional weight gain (kilograms gained within a specific time period/prepregnant weight) was measured in those same intervals. Fetal growth rate was significantly slower in the first and second periods, and significantly faster in the third time period for women with a low BMI, compared with those of average BMI. Fetal growth also significantly increased with increases in proportional weight gain in the second and third periods, but not in the first. Fetal growth appears to be independently associated with maternal BMI and proportional maternal weight gain.