It is normal for HCG to double every other day in the first trimester. The HCG in the body starts to rise significantly after the sperm and egg have united and the fertilized egg has been laid, and there is a difference in the rate of HCG increase between intrauterine and ectopic pregnancies. In normal intrauterine pregnancies, HCG increases more rapidly, by more than 60% at 24-hour intervals and can double at 48-hour intervals. In the case of ectopic pregnancy the growth rate of human chorionic gonadotropin cannot be doubled at an interval of 48 hours. In the absence of a gestational sac, the change in HCG can initially determine whether the pregnancy is intrauterine or ectopic. However, there are special cases of only general changes, and only a general judgment can be made without accurate diagnosis.