Human chorionic gonadotropin (HCG) is a glycoprotein secreted by trophoblast cells of the placenta. It is produced by α and β Glycoprotein composition of the dimer. Glycoprotein hormone with molecular weight of 36700, α The subunit is basically similar to FSH (follicle stimulating hormone), LH (luteinizing hormone) and TSH (thyroid stimulating hormone) secreted by the pituitary gland, so it can cross react with each other, while β The structures of subunits vary. After the fertilized eggs of mature women move to the uterine cavity for implantation, they form embryos. During the development and growth of the fetus, the syncytiotrophoblast cells of the placenta produce a large amount of hCG, which can be excreted into the urine through the maternal blood circulation. HCG levels in serum and urine increased rapidly at 1-2.5 weeks of gestation, peaked at the 8th week of gestation, and fell to a moderate level until the 4th month of gestation, and remained until the end of pregnancy.