Physiological studies of human chorionic gonadotropin and free subunits in the amniotic fluid compartment compared to those in maternal serum.

We measured intact hCG, free alpha hCG, and free beta hCG levels in amniotic fluid and maternal serum using specific monoclonal antibody-based immunoradiometric assays. Compared to maternal serum, amniotic fluid had low levels of intact hormone along with high levels of its free subunits. The mean amniotic fluid hCG level was 1 mg/L at 13 weeks, and it progressively decreased to 0.100 mg/L by the 23rd week. Amniotic fluid alpha hCG levels were highest at 15 and 16 weeks (0.340 mg/L) and rapidly declined to concentrations less than 0.050 mg/L at 21 weeks. Amniotic fluid beta hCG concentrations were about 0.200 mg/L between 13 and 16 weeks and progressively decreased until the 23rd week. In vitro stability studies of hCG and its subunits demonstrated that dissociation of hCG into its subunits was not responsible for these results. Both hCG and free subunits detected in amniotic fluid were indistinguishable from standard hCG and free subunit preparations, as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our findings are consistent with the hypothesis that the presence of hCG and free subunits in the amniotic fluid represents secretion from the trophoblastic tissue into the amniotic fluid compartment. Furthermore, high serum hCG levels in association with low levels of its free subunits are probably the result of polarized secretion of the intact hormone from the syncytiotrophoblasts into the maternal circulation.