Human chorionic gonadotropin (hcg) is a glycoprotein secreted by the trophoblast cells of the placenta and consists of two subunits, a- and b-. hCG enters the mother’s blood after fertilization and proliferates rapidly until the eighth week of pregnancy, then slowly decreases in concentration until the eighteenth to twentieth week, and then remains stable. Complete HCG is produced entirely by the syncytial trophoblast of the placental chorionic villi. Its main function is to stimulate the corpus luteum, which facilitates the continuous secretion of estrogen and progesterone to promote the formation of the uterine meconium and the growth and maturation of the placenta. Modern belief is that HCG is produced by trophoblast transitional cells and syncytiotrophoblast cells. It increases rapidly in value during the first 8 weeks of pregnancy to maintain the pregnancy. After about 8 weeks of gestation, HCG gradually decreases until it reaches relative stability at about 20 weeks. Normal reference values: weeks of gestation HCG (IU/L) 0.2-1 week 5-50 1-2 weeks 50-500 2-3 weeks 100-5000 3-4 weeks 500-10000 4-5 weeks 1000-50000 5-6 weeks 10000-100000 6-8 weeks 15000-200000 2-3 months 10000-100000