Human chorionic gonadotropin, or hCG, is a hormone involved in pregnancy. Normal adults do not have hCG in their bloodstreams, but pregnant women do. Because hCG helps maintain the physiological changes that support a pregnancy, it causes a cessation of menstruation as well as several other physiological effects.
Production of hCG
Under most circumstances, hCG comes only from one source---developing embryonic tissue. If you conceive, your fertilized egg begins to produce hCG within a few days of conception. The hormone communicates with your body, letting you know that you are pregnant. The presence of hCG signals your body to maintain the uterine lining that has built up during your menstrual cycle. This hormone is critical to pregnancy, because your fertilized egg needs to implant in the uterine lining.
Other Sources of hCG
Although hCG comes primarily from embryonic tissue, two other potential sources of hCG also exist. Some tumors can secrete hCG, which make the hormone a tumor marker in nonpregnant individuals. The only other circumstance in which hCG would appear in your bloodstream if you weren't pregnant is one in which your doctor is treating you with fertility drugs made of hCG, such as Pregnyl. The drug database RxList.com notes that Pregnyl helps to induce ovulation and may enhance fertility in some individuals.
The natural role of hCG in a woman's body is to prevent the shedding of the uterine lining, or menstruation. As such, hCG---regardless of source---limits or prevents menstruation. Although you may experience slight periodic vaginal bleeding during pregnancy, only very rarely would you have a full menstrual period while you're pregnant. Similarly, if you're undergoing treatment with hCG-containing drugs or have high levels of hCG due to a tumor, you won't menstruate.
Testing for hCG
Because hCG prevents menstruation, hCG levels during pregnancy start to rise well before you miss your first menstrual period. In her book "Conception, Pregnancy and Birth," Dr. Miriam Stoppard explains that you can typically test for hCG in your urine using a home pregnancy test several days before you actually miss a menstrual period. The kidneys filter hCG from the bloodstream into the urine, and home pregnancy tests detect it there.
If your hCG levels are very low during pregnancy, you may miscarry. Miscarriage isn't due directly to low hCG but relates instead to the hormone progesterone. If hCG is low, you're also likely to produce low levels of progesterone, which is directly responsible for maintaining the uterine lining. Your obstetrician may test for both hCG and progesterone levels early in your pregnancy to determine whether you're at risk for shedding your uterine lining and losing the pregnancy.