A study on the hyperinsulinism of late pregnancy

Much research has demonstrated that in late pregnancy glucose administration causes a marked increase of peripheral insulin levels. To ascertain whether this particular increase is due to increased insulin secretion and/or to reduced hepatic insulin removal, we measured blood glucose, plasma C-peptide and plasma insulin during OGTT in 7 nonpregnant women and in 20 nondiabetic women at third trimester of gestation and 60-90 days after delivery. The C-peptide/insulin molar ratio was calculated for all subjects. Data obtained showed that both plasma insulin and C-peptide response to oral glucose is considerably higher in women at third trimester of pregnancy as compared with that observed in the same subjects after delivery and in nonpregnant women. The basal (overnight fasting) C-peptide/insulin molar ratio did not differ significantly between pregnant and nonpregnant women. After the oral glucose load the molar ratio was sharply reduced in all groups of subjects, but the overall decrease in the pregnant women in the three hours following oral glucose was considerably greater than in postpartum and in nonpregnant women. The increased plasma C-peptide response clearly indicates that in pregnancy oral glucose-induced hyperinsulinism is caused by increased insulin release from pancreatic B-cells. Moreover, the marked overall decrease of the C-peptide/insulin molar ratio suggests, even if it does not definitely prove, that hyperinsulinism after glucose in late pregnancy may be a consequence not only of increased insulin secretion, but also of decreased hepatic extraction of insulin.