Ectopic pregnancy is a type of ectopic gestation, which refers to the implantation of the gestational sac outside the uterine cavity. Since the gestational sac in an ectopic pregnancy lacks a normal and safe growth environment, its growth and development will be restricted. Therefore, some of its characteristics are different from those of a normal intrauterine pregnancy. Among them, the change in hCG is the most important difference.Under normal circumstances, the characteristic of hCG in an intrauterine pregnancy is that it rises well and doubles every other day. In an ectopic pregnancy, the growth and development of the gestational sac are restricted, and the proliferation of trophoblast cells is not active enough. So, hCG rises slowly and fails to double every other day.In the early stage of pregnancy, the condition and diagnosis of an ectopic pregnancy can be roughly evaluated by detecting hCG. If hCG rises slowly and fails to double every other day, it should be determined that there may be an ectopic pregnancy, and it is necessary to combine with B - ultrasound. At the same time, hCG reflects the treatment situation of an ectopic pregnancy. If the treatment of an ectopic pregnancy is effective, hCG should decline significantly.