A change in the isoforms of human chorionic gonadotropin occurs around the 13th week of gestation.

hCG exhibits a considerable heterogeneity in blood during pregnancy. The overall charge of the isoforms of hCG has been shown to be more negative in the early than in the latter part of pregnancy. The present study analyzes the change in median charge and the charge heterogeneity as pregnancy progresses. The hCG activity in sera from 76 women from weeks 6-43 of gestation was measured with a noncompetitive time-resolved sandwich fluoroimmunoassay. The median charge and degree of charge heterogeneity of the isoforms of hCG in each serum was determined by electrophoresis in 0.10% agarose suspension. Median charge was expressed as median electrophoretic mobility. Electrophoresis with a high resolution revealed that the number of isoforms of hCG in a serum specimen from week 36 of gestation was 20-30. A change in median charge was found to occur at a limited time period of gestation, around the 13th week. All 16 sera from weeks 6-10 had isoforms of hCG with a more negative median charge than that of hCG in 21 sera from weeks 16-43 of gestation. The change in charge was accompanied by an increased degree of charge heterogeneity. There was a significant (P < 0.01) correlation between median mobility and the concentration of hCG during weeks 11-15 of gestation, but not before or after this period. There was no relationship between the sex of the fetus and the median charge of the isoforms of hCG. The data show that a change in the isoforms of hCG, revealed by a change in median charge of hCG, occurs at a limited time around the 13th week of gestation. This change occurs when the hCG concentration in blood decreases, and the placental production of estradiol and progesterone rapidly increases.