A drop in HCG is not necessarily a miscarriage and is determined by the duration of the pregnancy. In a normal pregnancy, HCG in the blood is elevated on the seventh day after ovulation and rises 1-fold every 1.7-2 days thereafter. It rises continuously until 8-10 weeks of pregnancy, but after 10 weeks, the HCG in the blood drops rapidly, and in the middle and late stages of pregnancy, the HCG is only 10% of the peak of pregnancy. Therefore, if the HCG in the blood drops before 10 weeks of pregnancy, it is to miscarry. If the HCG drops beyond 10 weeks, it is a normal physiological phenomenon, so the timing of the pregnancy must be used to determine if a miscarriage is going to occur.