HCG is short for human chorionic gonadotropin. This chemical is a hormone produced by fetal tissue, which helps communicate to a woman's body that she is pregnant and assists her body in making changes necessary to maintain the pregnancy. Only pregnant women have naturally occurring hCG in their blood and urine.
HCG Production
HCG, which is similar in structure to a hormone produced by the anterior pituitary gland and called luteinizing hormone, comes from a portion of embryonic and fetal tissue called the chorion, a part of which eventually becomes the placenta. Embryos begin producing hCG within just a few days of conception. Explains Dr. Miriam Stoppard in her book "Conception, Pregnancy and Birth," hCG production increases dramatically until around the third or fourth month of pregnancy, and then begins to decrease steadily through the remainder of pregnancy.
HCG Function
The purpose of hCG, explains Dr. Lauralee Sherwood in her book "Human Physiology," is to signal a woman's body that she is pregnant. When a woman ovulates each month, her uterine lining is thick and ready for a fertilized egg to implant. If fertilization doesn't occur, the uterine lining starts to slough off around 14 days after ovulation. If ovulation does occur, hCG from the developing embryo signals her body that she is pregnant, and helps to maintain the uterine lining.
Pregnancy Tests
Because embryos begin secreting hCG within just days of conception, the hormone starts to build up in a woman's blood very shortly after fertilization. Typically, early levels of hCG are too low to be detected, but around the time of her missed period, a woman can generally use a home pregnancy test to determine whether there's hCG in her urine, which is a positive indicator of pregnancy. Only pregnant women have hCG in their urine, notes Dr. Stoppard.
Pharmaceuticals
Because of its structural similarity to luteinizing hormone, hCG is sometimes administered pharmaceutically to induce ovulation, which is normally induced by luteinizing hormone. A pharmaceutical compound called Pregnyl, which comes from hCG isolated from the urine of a pregnant woman, helps women who have difficulty ovulating to ovulate, and increases fertility in some cases. Women who are taking Pregnyl may test positive for pregnancy, even if they're not pregnant.
Expert Insight
While pregnancy tests can indicate whether a woman is pregnant, home tests can't quantify the amount of hCG in the urine. Explains Dr. Stoppard, some obstetricians like to test the blood of pregnant women to ensure that their hCG levels are rising appropriately during pregnancy -- hCG should increase predictably and double every few days. If it does not, this may indicate a problem with the pregnancy.
References
- "Conception, Pregnancy and Birth"; Miriam Stoppard, M.D.; 2008
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- RXList: Pregnyl
