When the trophoblast of the fertilized egg is formed (day 6 after fertilization), it begins to secrete trace amounts of HCG, which can be detected in maternal blood 10 days after fertilization. Serum β-hCG levels rise from 5 IU/L to 50 IU/L within 1 week of implantation of the fertilized egg. It is about 100 IU/L 14 days after ovulation. During the first 6 weeks of normal pregnancy, HCG levels double in about 36 to 48 hours. After 6 weeks of pregnancy, when the HCG level reaches 6000 to 10000 IU/L, the HCG rise starts to slow down. HCG reaches its peak at 8-10 weeks of gestation, about 100000-200000 IU/L, and continues to decline rapidly after 10 weeks (about 1-2 weeks), dropping to the lowest value at about 20 weeks of gestation and continuing until delivery; it decreases significantly after delivery, and drops to normal level about 2 weeks after delivery if there is no placental residue. In middle and late pregnancies, the blood HCG concentration is about 10% of the peak. In spontaneous abortion and ectopic pregnancy, hCG levels are usually low. Maternal serum HCG levels are abnormally high in multiple pregnancies, singleton pregnancies with hemolysis of Rh blood type incompatibility, chylothorax, or choriocarcinoma. Maternal serum Free-HCG levels are also abnormally high in fetuses with Down syndrome in the middle of pregnancy; 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 and the prognosis can be judged by the multiplication characteristics. When the initial HCG level is lower than 2000 IU/L, if it is a normal intrauterine pregnancy, most of the HCG levels at 48 hours multiply; if the increase of HCG level at 48 hours is less than 50%, and the HCG level still does not reach 2000 IU/L, it indicates embryonic death. Usually, in complete miscarriage, the HCG level decreases significantly (more than 50% decrease in HCG level at 48 hours). In case of gravidity, the trophoblast cells are highly proliferated and produce a large amount of HCG, and the serum HCG titer is usually higher than the normal pregnancy value at the corresponding gestational week; moreover, after 12 weeks of menopause, it continues to rise continuously as the uterus enlarges, with HCG above 10,000 IU/L, often exceeding 100,000 IU/L, and it continues not to fall, using this difference as an aid to diagnosis. Under normal circumstances, after evacuation of the gravida, HCG decreases steadily, and the average time to first decrease to normal is about 9 weeks, and the longest is not more than 14 weeks. If the HCG level continues to be high for more than 9 weeks after evacuation of the gravida, or for more than 4 weeks after miscarriage, term delivery, or ectopic pregnancy, or if it once dropped and then rose again, the diagnosis of trophoblastic neoplasm can be made when combined with clinical manifestations, excluding residual pregnancy or re-pregnancy. If the HCG level doubles normally, when the HCG level reaches 1000-1800 IU/L, vaginal ultrasonography can show most intrauterine pregnancies, and 2-4 mm liquid dark area (gestational sac) can be seen in the uterine cavity. β-hCG 1800-2300 IU/L, transvaginal ultrasonography can show 100% intrauterine gestational sac. Pregnancy failure can be predicted according to the change in β-hCG. β-hCG ratio (β-hCG48h:0h) <0.87 (or β-hCG decrease >13%) has 92.7% sensitivity and 96.7% specificity for predicting pregnancy failure. β-hCG ratio >2 has 77.2% sensitivity and 95.8% specificity for predicting an unlocated pregnancy that ends up as a live intrauterine pregnancy, PPV was 86.6% and NPV 90.9%. On the 16th to 18th day after artificial insemination, if the HCG level can reach 300 IU/mL, the chance of obtaining a live fetus is 88%; if the HCG level is <300 IU/mL, the chance of obtaining a live fetus decreases to 22%. In ectopic pregnancy, the HCG value is usually lower than in normal pregnancy. The risk of ectopic pregnancy increases if HCG is measured dynamically, if there is no vaginal bleeding, if HCG rises less than 50% at 48 hours, or if blood HCG falls slowly with a half-life greater than 1.4 days; if β-HCG is >2000 IU/L and the gestational sac is not detected in the uterine cavity by vaginal ultrasound, ectopic pregnancy is mostly diagnosed. The presence of HCG during non-pregnancy indicates the presence of tumors that secrete this hormone directly or ectopically, such as staphyloma, erosive staphyloma, choriocarcinoma, ovarian immature teratoma, ovarian asexual cell tumor, ovarian adenocarcinoma, hypothalamic choriocarcinoma, hepatic embryonal tumor, hepatocellular carcinoma, intestinal cancer, pancreatic cancer, gastric cancer, lung cancer, breast cancer, kidney cancer, etc.