Beta HCG levels rise geometrically in the serum in the first 8 weeks of pregnancy. Detectable amounts of beta HCG are present 8-11 days after conception. During the second to fifth week, HCG levels double in about 1.5 days. After 5 weeks of gestation, the doubling time gradually increases to 2-3 days. Serial determination of HCG is helpful when abnormal pregnancy is suspected. In ectopic pregnancy and spontaneous abortion HCG concentration increases slowly or decreases. Ultrasonography should detect a gestational sac in the uterus of all patients having HCG concentration > 6500 mIU/mL. Failure to detect a gestational sac 24 days or more after conception is presumptive evidence of ectopic pregnancy. The presence of twins approximately doubles the HCG concentration.