Human chorionic gonadotropin (HCG), a glycoprotein hormone secreted by placental syncytiotrophoblast cells, is found in pregnant women's blood, urine, colostrum, amniotic fluid and fetus.

　　On the 6th day after fertilization, the trophoblast layer of the fertilized egg was formed and HCG was secreted. After implantation, HCG was detected in maternal blood with specific -HCG antiserum. HCG secretion increases very quickly in the early gestation, and reaches the peak at 8 ~ 10 weeks of gestation, and then decreases rapidly after 1-2 weeks of gestation, and the serum HCG concentration in the third trimester is only 10% of the peak, and continues until delivery. If no placenta remains after delivery, it will disappear within 2 weeks after delivery.

　　HCG is the only placental hormone that does not increase with the increase of placental weight. After secretion, it directly enters maternal blood and hardly enters fetal blood circulation. HCG can be excreted into the urine through the pregnant woman's blood circulation, and the serum HCG concentration is slightly higher than that of the urine, and the relationship is parallel.