Free and protein-bound steroids in amniotic fluid of midpregnancy

Ninety-seven amniotic fluid (AF) specimens, 67 with male fetuses and 30 with female fetuses, were obtained by amniocentesis at 14 to 18 weeks of gestation for analysis of free and protein-bound estrone (E1), estradiol (E2), androstenedione (A), testosterone (T), and progesterone (P). Ten maternal plasma (MP) samples of comparable gestational age were analyzed for the same free and protein-bound steroids. Significantly lower total E1, E2, A, and T, but not P, levels were found in AF when compared to MP. The percentage of free steroids was higher in AF than in MP, while the percentage of specific protein-bound steroids was lower in the AF than in MP. Free (unbound) steroid levels of E1 and P were higher in AF than in MP, whereas levels of E2 were lower in AF than in MP. The free A and T levels in MP were lower than in AF with male fetuses, yet higher than in AF with female fetuses. Possible biologic activity of steroids in this fluid may depend on the dynamic balance between the free, the specific protein-bound, and the nonspecific protein-bound steroids.