HCG is a glycoprotein hormone produced by the placenta in pregnant women, which can stimulate the transformation of ovarian corpus luteum into gestational corpus luteum to secrete large amounts of progesterone to maintain pregnancy, and also transform cholesterol or pregnenolone in the blood into progesterone. Adequate HCG in pregnant women can keep the corpus luteum of pregnancy from degenerating and maintain embryonic development, and vice versa, it can affect embryonic development or implantation. The level of HCG increases with the number of days of pregnancy and shows a rate of doubling every other day. HCG measurements vary very much between pregnancies and there is no constant standard. Generally, at 4 weeks of pregnancy, two weeks after ovulation, HCG is between 50-500 IU/L; at about 8 weeks of pregnancy, HCG reaches its peak, up to about 200,000 IU/L; then it gradually decreases and remains at a constant level until the third trimester. To sum up, HCG is a dynamic indicator, and it is difficult to determine ectopic pregnancy or pre-term abortion by the result of a single HCG, which requires dynamic observation and close follow-up. Progesterone has a certain degree of stability, and it is recommended that combined monitoring of serum HCG as well as progesterone, combined with ultrasound, can determine the outcome of pregnancy earlier and more accurately.