Human chorionic gonadotrophin enters the mother's blood after fertilization and proliferates rapidly until the 8th week of pregnancy, then slowly decreases in concentration until the 18th to 20th week, and then remains stable. In an average intact human, chorionic gonadotropin levels rise to at least 2.5 MoM before being associated with down syndrome. However, the average level of human chorionic gonadotropin in the mother blood of DS patients was only 1.3MoM, which could not clearly identify DS patients. The human chorionic gonadotropin-related molecules used for DS examination had free b-hcg and high glucose hCG (h-hcg).

　　The levels of human chorionic gonadotropin-related molecules are also related to the sex of the fetus.

　　The complete human chorionic gonadotropin is produced entirely from the syncytiotrophoblast of the placental chorionic membrane. Human chorionic gonadotropin is produced by trophoblastic transitional cells and somatic cells. Human chorionic gonadotropin is secreted from 10 to 14 days after conception.