Trace amounts of HCG begin to be secreted at the time of formation of the trophoblastic layer of the fertilized egg (day 6 after fertilization) and can be detected in maternal blood 10 days after fertilization. Within 1 week of implantation of the fertilized egg, the serum β-hCG level rises from 5 IU/L to 50 IU/L, and about 100 IU/L on the 14th day after ovulation.During the first 6 weeks of normal pregnancy, the HCG level doubles in about 36~48 hours; the rate of increase of HCG begins to slow down after 6 weeks of pregnancy when the HCG level reaches 6,000~10,000 IU/L.HCG reaches a HCG reaches its peak at 8~10 weeks of pregnancy, about 100000~200000IU/L, and then decreases rapidly after 10 days (about 1~2 weeks), and then decreases to the lowest value at about 20 weeks of pregnancy, and then continues until delivery; it decreases significantly after delivery, and then decreases to the normal level about 2 weeks after delivery if there is no placenta left after delivery. In middle and late pregnancy, blood HCG concentration is about 10% of the peak. In spontaneous abortion and ectopic pregnancy, hCG level is usually low. Maternal serum HCG levels are abnormally high in multiple pregnancies, singleton pregnancies with Rh blood group incompatibility hemolysis, hyperemesis gravidarum, or choriocarcinoma. Maternal serum Free-HCG levels are also abnormally high in Down syndrome fetuses in mid-gestation; therefore, HCG can be used as a serum biochemical marker for prenatal screening. In early pregnancy, HCG increases rapidly, with a doubling time of about 1.4 to 2.2 days. It is generally believed that in normal intrauterine pregnancy, the serum β-hCG level increases by a minimum or at least 24% per day, and by at least 53% in 2 days, so the β-hCG level can be measured dynamically in early pregnancy to determine the prognosis by using the multiplication characteristics. When the initial HCG level is lower than 2000IU/L, if it is a normal intrauterine pregnancy, most of the 48-hour HCG levels multiply; if the 48-hour increase in HCG level is less than 50%, and the HCG level still does not reach 2000IU/L, it suggests that the embryo is dead. Usually, in complete miscarriage, there is a significant decrease in HCG levels (more than 50% decrease in HCG levels at 48 hours). In gravidas, trophoblast cells are highly proliferative, producing large amounts of HCG, and serum HCG titers are usually higher than normal pregnancy values for the corresponding gestational week; moreover, after 12 weeks of menopause, as the uterus continues to increase continuously with uterine enlargement, HCG is above 10,000 IU/L, and often exceeds 100,000 IU/L, and continues to be unabated, and the use of this difference aids in the diagnosis. Under normal circumstances, after evacuation of the gravidarium, HCG declines steadily, with the average time to first decrease to normal being about 9 weeks and up to 14 weeks. If the gravidas is evacuated for more than 9 weeks, or if the blood HCG level is persistently high for more than 4 weeks after miscarriage, full-term delivery, or ectopic pregnancy, or if it rises again after having fallen once, and if we exclude residual pregnancy or re-pregnancy, and in combination with the clinical manifestations, we can diagnose trophoblastic neoplasia. If HCG level doubles normally, when HCG level reaches 1000~1800IU/L, vaginal ultrasound can show most of intrauterine pregnancy, and 2~4mm liquid dark area (gestational sac) can be seen in the uterine cavity. β-hCG1800~2300IU/L, transvaginal ultrasound can show 100% of intrauterine gestational sac. Pregnancy failure could be predicted according to the change of β-hCG. β-hCG ratio (β-hCG48h:0h) <0.87 (or β-hCG decrease >13%) had a sensitivity of 92.7% and a specificity of 96.7% for predicting pregnancy failure. β-hCG ratio >2 had a sensitivity of 77.2% and a specificity of 95.8% for predicting that a pregnancy with an undetermined location ended up as an intrauterine viable pregnancy, PPV86.6%, and NPV90.9%. On days 16-18 after insemination, if the HCG level could reach 300 IU/mL, the chance of obtaining a live fetus was 88%; if the HCG level was <300 IU/mL, the chance of obtaining a live fetus decreased to 22%. In ectopic pregnancies, HCG values are usually lower than in normal pregnancies. Dynamic measurement of HCG, if there is no vaginal bleeding, the rise of HCG is less than 50% in 48 hours, or the decline of blood HCG is slow and the half-life is more than 1.4 days, the risk of ectopic pregnancy is increased; if β-HCG>2000IU/L, vaginal ultrasound did not detect the gestational sac inside the uterine cavity, most of them can be diagnosed as ectopic pregnancy. The presence of HCG in non-pregnancy suggests the existence of tumors that directly or ectopically secrete this hormone, such as gravidarium, erosive gravidarium, choriocarcinoma, immature teratoma of the ovary, anaplastic tumor of the ovary, ovarian adenocarcinoma, hypothalamic chorionic villus tumor, hepatic embryonic tumors, hepatocellular carcinoma, intestinal carcinoma, pancreatic carcinoma, gastric carcinoma, lung carcinoma, breast carcinoma, renal carcinoma, and so on.