The hormone human chorionic gonadotropin, or hCG, is naturally produced early in pregnancy. Women do not produce hCG on their own; the cells surrounding the embryo produce the hormone, so it is a reliable marker of pregnancy. The hCG hormone maintains the production of progesterone by the ovaries and helps the embryo implant in the uterus.
Natural Pregnancy
Shortly after conception, the earliest cell divisions produce a hollow ball called a blastocyst. The outer cells, called the syncytiotrophoblast, eventually become the embryo's contribution to the placenta while the inner cell mass goes on to form all of the tissues of the baby. The syncytiotrophoblast produces hCG, whose primary role is to tell the ovaries to continue producing progesterone, the hormone that maintains the lining of the uterus in which the embryo will implant.
Ovarian Effects
The hCG hormone acts on the corpus luteum, the ovarian structure that forms after a follicle releases its egg. In a normal menstrual cycle, the corpus luteum produces progesterone that causes the lining of the uterus to build up. If no pregnancy occurs, the corpus luteum regresses, progesterone levels fall and the body sheds the uterine lining during menstruation. When a pregnancy occurs, hCG binds to special receptors in the corpus luteum, "rescuing" the corpus luteum from regression. Its continued production of progesterone maintains the uterine lining so that the embryo can implant.
Uterine Effects
According to the authors of "Obstetrics: Normal and Problem Pregnancies," hCG promotes communication between the cells of the outermost layer of the embryo and the cells of the uterine lining. The hCG hormone and other substances allow the formation of cellular channels called gap junctions, tube-like channels that allow materials to flow directly from the inside of one cell to the inside of another. Cell-to-cell communication through gap junctions must occur before the two cell types can fuse together, firmly implanting the embryo.
Fertility Treatment
The fertility drugs Ovidrel and Pregnyl contain hCG, says the Mayo Clinic. These drugs stimulate ovulation because hCG is structurally very similar to luteinizing hormone, the hormone that normally stimulates ovulation during a menstrual cycle.
Pregnancy Test
Scientists took advantage of the pregnancy-specific production of hCG to develop quick and easy pregnancy tests. The urine of pregnant women contains a measurable amount of hCG as early as 12 to 14 days after conception, says the American Pregnancy Association. With the modern over-the-counter pregnancy tests, hCG in the urine chemically reacts with test substances to produce a color change that indicates pregnancy. Because only the cells surrounding an embryo produce measurable amounts of hCG, its presence in the urine indicates that a woman must be pregnant.
References
- "Reproduction": The human corpus luteum
- "Obstetrics: Normal and Problem Pregnancies;" Steven Gabbe, Jennifer Niebyl and Joe Leigh Simpson; 2007
- MayoClinic.com: Infertility: Treatments and drugs
- American Pregnancy Association: Human Chorionic Gonadotropin (hCG): The Pregnancy Hormone
