Appearance of hCG in pregnancy plasma following the initiation of implantation of the blastocyst.

Plasma chorionic gonadotropin levels were measured by three different assay methods during early pregnancy in four patients following induction of ovulation with Pergonal and hCG. Radioligand-receptor assay of unextracted samples was subject to non-specific interference by plasma proteins, causing an apparent elevation of gonadotropin levels during the first few days after fertilization. By contrast, the gonadotropin values measured by a highly sensitive LH/hCG bioassay were consistent with those obtained with a sensitive and specific radioimmunoassay for hCG, and showed that the first significant rise in plasma hCG occurred 9 to 13 days after ovulation. These results indicate that hCG does not appear in the maternal circulation until after the initiation of implantation of the blastocyst.