Plasma concentrations of human chorionic gonadotropin from the time of implantation until the second week of pregnancy

A systematic study of the daily concentrations of human chorionic gonadotropin (hCG) in maternal circulation following the time of probable implantation in 19 normal pregnancies was carried out. With the use of a specific antiserum to hCG β-subunit, significant increases in circulating hCG were observed by day 8 following the luteinizing hormone (LH) peak in 5.3% of cycles, by day 9 in 15.8%, by day 10 in 532%, and by day 11 in 100% of the cycles resulting in a normal pregnancy. After an initial rapid rise, mean plasma concentrations rose exponentially, with a doubling time of 1.3days, to reach concentrations between 50 and 250IU/l at the time of the first missed period. The time of detection of hCG was in close agreement with the anticipated time for this event estimated from morphologic studies. In three abnormal pregnancies, all of which ended in spontaneous abortion, hCG concentrations became progressively more abnormal from within 2 to 5days of the appearance of this hormone in maternal plasma. Despite this, actual abortion did not occur for several days, or in one case for several weeks, after the divergence of hCG concentration from the normal range.