Levels of free and protein-bound progesterone in plasma during late pregnancy

Using a filter method, levels of free (unbound) and protein-bound progesterone in blood samples were measured serially from the 23rd week of pregnancy until delivery in 16 healthy women. In addition, total estrogen and progesterone concentrations were determined by radioimmunoassay. The levels of both free and total progesterone increased steadily until the end of pregnancy. Free progesterone increased with advancing pregnancy more in proportion to the total progesterone, as it rose from 6% (of total) at week 24 to 13% at week 40. Both free and total progesterone decreased markedly very soon (2 h) after delivery. However, the corresponding decrease in the level of free progesterone was considerably less, with the result that it rose to 19% of the total, the greatest proportion of free progesterone observed at any time. The levels of both total estrogen and progesterone in plasma increased with advancing pregnancy, and at no time, prior to delivery, was there a significant fall in progesterone or an abrupt rise in estrogen.