The impact of an increasing fetal and placental mass on maternal glucose disposal rate (GDR) was studied in 17 pregnant women. Eleven pregnancies were singleton pregnancies, five were twin and one was a triplet pregnancy. Both the maternal fasting glucose concentration [Glc] and the total fetal and placental weight (W) were significantly correlated with an increased maternal GDR. Glucose concentration and conceptus weight were interdependent. The multiple linear regression of glucose disposal rate on glucose concentration and total weight is given by GDR = -1.573 + 0.176W + 0.932[Glc] +/- 0.24, r2 = 0.52, P < 0.01. These data are utilized to estimate the glucose utilization rate of the human conceptus at a fasting glucose concentration of 3.9 mM with a range of 10-15 mg.kg-1.min-1. Maternal GDR increases with increasing glucose concentration at a rate similar to that reported for newborn infants, consistent with a high glucose utilization by the uterus.