Human chorionic gonadotropin is a hormone produced by the cells of the placenta during pregnancy. HCG enters the bloodsteam when a fertilized egg implants into the uterus, usually about six days after fertilization.
In most pregnancies, hCG levels double every 48 to 72 hours, peaking between eight and 11 weeks of pregnancy before beginning to decrease.
At four weeks of pregnancy, calculating from the date of the last menstrual period, typical hCG levels range from 5 to 426 mIU/ml, according to American Pregnancy.
Low hCG levels can signal an impending miscarriage or ectopic pregnancy, but can also occur in a healthy, problem-free pregnancy.
High hCG levels may indicate a multiple pregnancy or a molar pregnancy but are not necessarily an indication of a problem or unusual circumstances.
Because of the wide variations from one woman to another, hCG is not usually used to date a pregnancy.