Maternal serum levels of sex steroid-binding protein during pregnancy.

Serum concentrations of sex steroid-binding protein (SBP) were measured by an immunodiffusion technique in a total of 133 samples from 70 healthy pregnant women, 18 puerperal women and 21 non-pregnant women. In 6 individuals, trends or serum SBP were established from multiple samples taken throughout pregnancy. Maternal serum SBP levels had increased significantly above base line values as early as by 10 weeks gestation, and attained a plateau between 25 and 30 weeks, at which time mean values were about 10-fold higher than the normal non-pregnant range, the levels then declined slightly towards term. Parallel measurements of dihydrotestosterone-binding capacity of SBP showed a good correlation between SBP content and biological activity. Six abnormal pregnancies were encountered in the present study, and in five of them the molar ratios of bound hormone to SBP concentration were much lower than the normal value, suggesting a reduced binding capacity of the protein for the hormone.