The abbreviation hCG is short for human chorionic gonadotropin. It is a hormone produced by a developing embryo during pregnancy. The hormone has an important role in both the detection and the maintenance of pregnancy. Pregnant women have very high hCG levels, but nonpregnant women and men do not have the hormone in their bloodstreams.
The hormone hCG, explains Lauralee Sherwood, Ph.D., in her book "Human Physiology," is a glycoprotein, meaning that it is composed of both sugar and protein and binds to receptors on cells. Once bound to the cells, it sends signals to cells across the cell membrane. The hormone isn't produced by all adults. Instead, embryonic cells begin producing hCG shortly after a woman conceives. Once the embryonic cells differentiate into tissue types, the tissues of the placenta take over hCG production.
The purpose of hCG is to maintain an important temporary maternal endocrine cell group called the corpus luteum. Explains Gary Thibodeau, Ph.D., in his book "Anatomy and Physiology," when a woman ovulates and releases an egg, the cells that surrounded the egg while it was in the ovary become hormone-secreting cells collectively called the corpus luteum. If an egg isn't fertilized, its corpus luteum dies within about 14 days. In the case of fertilization, hCG maintains the corpus luteum.
By maintaining the corpus luteum that surrounds the egg, hCG also maintains the lining of the uterus, which is necessary to sustain a pregnancy if an egg is fertilized. When the corpus luteum surrounding an unfertilized egg dies, falling hormone levels cause the uterine lining to slough, which results in a menstrual period. By maintaining the uterine lining, hCG provides a place for embryonic development. The embryo is totally dependent on the uterine lining until the placenta is fully formed, usually during the fourth month of pregnancy.
Although hCG's primary role in pregnancy relates to the developing embryo, it has a separate utility—physicians can test for hCG levels in blood to monitor the viability of an early pregnancy. Also, home pregnancy tests can detect hCG in urine to detect early pregnancies. Explain Heidi Murkoff and Sharon Mazel in their book "What To Expect When You're Expecting," because the embryo begins to produce hCG soon after conception, women can test positive for pregnancy owing to to hCG production several days before a missed period.
Because of its similarity to another hormone, luteinizing hormone or LH, hCG mimics some of the effects of LH in the female body, explains Dr. Miriam Stoppard in her book "Conception, Pregnancy and Birth." LH normally stimulates ovulation, but if LH levels are low, injections of hCG have similar stimulatory effects. In women whose ovaries ripen eggs but don't release them, doses of hCG can stimulate ovulation and help improve fertility.