Total and free testosterone during pregnancy.

Total and free testosterone levels were measured throughout pregnancy and were compared with values found in nonpregnant women to determine whether the increase in plasma testosterone levels during pregnancy can be accounted for entirely by an increase in sex hormone-binding globulin or by an increase in testosterone production that would be reflected by an elevation in plasma free testosterone. The study employed a practical and precise method of determining free testosterone by means of equilibrium dialysis. Even though the testosterone level increased significantly throughout pregnancy, the free testosterone level remained within the range for nonpregnant women until week 28, after which time it was significantly elevated. This indicates that the increase in the total testosterone level before week 28 is due to a decrease in metabolic clearance rate, but after week 28 the production rate is increased. The source of this increase is unknown, but it might be fetal.