Chorionic gonadotropin concentration in early human pregnancy: comparison of specific and nonspecific assays

Daily blood samples were collected during 10 menstrual cycles in which conception had occurred, and changes in circulating concentrations of human chorionic gonadotropin (hCG) were measured simultaneously in specific and nonspecific radioimmunoassays. The hCG concentration profiles were not identical, and the fundamental differences could not be related to differences in technique. Both specific and nonspecific parameters of hCG concentration rose exponentially for the first 2 to 3 weeks following first detection and were thus amenable to linear regression analysis. Doubling times were significantly different at 2.3 and 1.8 days, respectively. Extrapolation of the regression data gave a (theoretical) plasma concentration of 3 IU/liter at 9.5 days after the luteinizing hormone peak using the specific assay but at only 5.5 days using the nonspecific assay. This difference is sufficiently large to suggest the presence of other forms of hCG or its subunits about the time of implantation and during early pregnancy.