Progesterone levels in amniotic fluid and plasma from women. I. Levels during normal pregnancy.

Abstract

Amniotic fluid samples from 45 early and 46 late pregnancies have been assayed for progesterone using a protein binding technique. Only 100 μl of amniotic fluid was needed for the determination. Simultaneous samples of plasma or serum have also been assayed. The highest progesterone levels in amniotic fluid were found between the 13th and 16th week of pregnancy when the levels were higher than in plasma. After this time there was a steady decline in amniotic fluid levels throughout pregnancy. The mean level during the 13th‐16th week period was 55.3 ng/ml amniotic fluid and 26.4 ng/ml between the 37th and 40th week. In contrast plasma levels of progesterone increased with advancing pregnancy and exceeded amniotic fluid levels from the 21st‐24th week onwards. The decrease in amniotic fluid progesterone concentration may be due to a relative decrease in placental progesterone synthesis and an increasing resistance to the passage of progesterone into the amniotic cavity.