After fertilization of the egg, human chorionic gonadotropin (HCG) is secreted by the syncytiotrophoblast cells of the chorionic trophoblast layer. serum HCG concentrations at different periods are as follows: i. greater than 5.0 U/L on days 7-10 of pregnancy; ii. greater than 100 U/L on days 30 of pregnancy; iii. greater than 2000 U/L on days 40 of pregnancy; iv. greater than 100,000 U/L in trophoblastic disease. dynamic changes in blood HCG can differentially diagnose intrauterine pregnancy or ectopic pregnancy. In ectopic pregnancy, the serum HCG level is lower than that of intrauterine pregnancy. If the doubling time is greater than 7 days, the possibility of ectopic pregnancy is very high; if the doubling time is less than 1.4 days, the possibility of ectopic pregnancy is very low.