Human chorionic gonadotropin (HCG) is a hormone that is normally produced first by the cells that make up the placenta, then later by the placenta during pregnancy. Its primary function is to support the pregnancy by encouraging the production of progesterone. This supports and promotes the further development of the placenta early in pregnancy. HCG sees a rapid increase early in pregnancy, but tapers off slowly as the placenta can produce enough progesterone to support the pregnancy on its own. The hormone also serves in aiding in the development of gonads in the fetus and the production of androgens by the testes of a male fetus.

Levels of HCG can first be detected approximately 11 days following conception, in a blood test. One to three days later, HCG can be detected with a urine test. Some highly sensitive tests can detect HCG as early as a week following ovulation. Normal home pregnancy tests are not able to detect HCG in the blood until at least 12 to 14 days after ovulation.

A typical pregnancy should have levels of HCG doubling every 48 to 72 hours up to the eighth to 11th week, at which time they start to stabilize and remain constant for the duration of the pregnancy. This occurs in about 85% of pregnancies. When HCG levels become high, they will start to double every 96 hours.

Pregnant women, especially those who have experienced fertility issues, place a lot of importance on HCG levels. Due to vast ranges of what can be normal, doctors advise women to not take them too seriously. Some women have low HCG levels and go on to have a healthy, normal baby.

Levels of the hormone in the blood are measured in milli-international units per millimeter (mIU/ml). Anything under 5mIU/ml registers as negative on a pregnancy test, while an HCG level over 25mIU/ml is positive. Ultrasounds have proven much more accurate in diagnosing and predicting the viability of a pregnancy than HCG levels. HCG levels can be tested in two ways: qualitative tests determine whether HCG exists in the blood or urine, while quantitative tests measure how much HCG is in the bloodstream.

A gestational sac usually completes its formation once HCG levels reach 1,200 mIU/ml. If an HCG reading is low or falling, it could signal a more recent pregnancy than previously thought, a mother who has had or is experiencing a miscarriage or blighted ovum, or an ectopic pregnancy. Higher levels than what is considered normal for the age of the pregnancy can mean that the pregnancy is further along than estimated, that there is a molar pregnancy present, or that multiple pregnancy or ovarian cancer exists.

After a miscarriage, HCG levels fall back to the pre-pregnancy range, which is less than 5.0 mIU/ml, about four to six weeks later. In some fertility treatment regimens, women may receive HCG injections to encourage ovulation or to extend the luteal phase of the cycle. When HCG is given during pregnancy, its purpose is to stimulate progesterone to further support the pregnancy. The effectiveness of this practice, however, is controversial at this time.