Serum human chorionic gonadotropin levels throughout normal pregnancy.

Human chorionic gonadotropin (hCG) levels were measured in the sera of 443 pregnant women by the beta-hCG radioimmunoassay in order to determine if the third-trimester secondary peak in hCG levels observed by less specific immunoassays was due to cross-reacting substances. hCG was detected as early as six days after presumed conception and peaked between 56 and 68 days, with a nadir at 18 weeks. No secondary rise in hCG levels was demonstrated, indicating that the nonspecific hCG immunoassays give spuriously high values for hCG during the last trimester of pregnancy.