Objective The purpose was to investigate to what extent fetal growth in non-diabetic pregnant women can be explained by rates of maternal energy substrate production and resting energy expenditure.

Methods Twenty non-smoking pregnant women with normal glucose tolerance were investigated at 36 weeks of gestation. Lipolysis and glucose production, maternal body composition, insulin resistance and resting energy expenditure were assessed. The data were related to estimated fetal weights.

Results Median (range) rate of glucose production was 805 (653–1337) μmol/min and that of glycerol production, reflecting lipolysis, was 214 (110–576) μmol/min. Mean resting energy expenditure was 1910±310 kcal/day. HOMA insulin resistance averaged 1.5±0.75. Estimated fetal weights ranged between 2670 and 4175 g. Multiple linear regression analysis showed that maternal fat mass explained 36% of the variation in insulin resistance, in turn accounting for 62% of the variation in the rate of glucose production. Further, glucose production explained 31% of the variation in fetal weight. Resting energy expenditure accounted for 51% of the variation in estimated fetal weight. Maternal BMI, insulin resistance, and glucose production explained 87% of the variation in resting energy expenditure.

Conclusion Our data indicates that fetal weight depends on the rate of glucose production, in turn determined by the degree of insulin resistance, which in part is induced by the maternal fat mass. Additionally, the variation in maternal resting energy expenditure, which was closely related to estimated fetal weight, could be explained to almost 90% by BMI, insulin resistance and glucose production.