Transplacental passage of insulin in pregnant women with insulin-dependent diabetes mellitus. Its role in fetal macrosomia.

BACKGROUND AND METHODS: Fetal macrosomia occurs despite nearly normal maternal blood glucose levels in women with diabetes treated with insulin. We examined the hypothesis that it may be caused by insulin transferred as an insulin-antibody complex from the mother to her fetus. We adapted and validated a method based on high-performance liquid chromatography and used it to quantitate insulin in small volumes (0.5 to 1.0 ml) of cord serum from 51 infants born to mothers with insulin-dependent diabetes mellitus.
RESULTS: In mothers receiving only human insulin (n = 6), only human insulin was detected in cord serum. Of the remaining 45 infants, whose mothers received animal insulin during pregnancy, 28 (group 1) had levels of animal (bovine or porcine) insulin (mean [+/- SE], 707 +/- 163 pmol per liter) that constituted 27.4 +/- 2.5 percent of the total insulin concentration (2393 +/- 500 pmol per liter) measured in the cord serum. The cord-serum insulin concentration in the remaining 17 infants (group 2), in whom only human insulin was detected (381 +/- 56 pmol per liter), was only 15 percent of that in group 1 (P less than 0.001). There was a significant correlation between the maternal and the cord-serum concentrations of anti-insulin antibody and the concentration of animal insulin in the baby (r = 0.77, P less than 0.01, and r = 0.76, P less than 0.001, respectively), suggesting that the animal insulin was transferred as an insulin-antibody complex. In group 1 the mean concentration of animal insulin in cord serum was higher in the 12 infants with macrosomia than in the 16 infants without the condition (1113 +/- 321 vs. 402 +/- 110 pmol per liter; P less than 0.05), and the concentration of animal insulin in cord serum correlated with birth weight (r = 0.39, P less than 0.05). The maternal glycosylated hemoglobin values and the incidence of respiratory distress syndrome were similar in groups 1 and 2.
CONCLUSIONS: Considerable amounts of antibody-bound insulin are transferred from mother to fetus during pregnancy in some women with insulin-dependent diabetes mellitus; the extent of transfer correlates with the maternal concentration of anti-insulin antibody. The correlation between macrosomia and the concentrations of animal insulin in cord serum indicates that the transferred insulin has biologic activity and suggests that the formation of antibody to insulin in the mother is a determinant of fetal outcome independent of maternal blood glucose levels.