HCG is short for human chorionic gonadotropin, which is an important reproductive hormone. Neither adult men nor adult women produce hCG; instead, it's produced only by fertilized eggs and embryos. The major purpose of the hormone is to communicate to a woman's body that she is pregnant, and to help her body maintain the pregnancy.
Female Reproduction
During your normal monthly reproductive cycle, one of your ovaries ripens and releases an egg. Once this happens, specialized tissue called the corpus luteum -- found in the ovary that released the egg -- starts secreting estrogen and progesterone. These hormones cause your uterine lining to thicken, which gives the egg a place to implant in the event that it's fertilized. If no fertilization occurs, the corpus luteum will die after about 14 days, and you'll menstruate.
HCG Secretion
If you conceive, however, one important job of the fertilized egg is to signal to your body that it's there, so that you don't menstruate and shed the uterine lining. The egg accomplishes this by secreting hCG, which communicates with the corpus luteum and maintains this tissue. As such, explains Dr. Lauralee Sherwood in her book "Human Physiology," your uterine lining remains thick, allowing the fertilized egg to implant and begin to develop.
Continued hCG Production
A developing egg -- which is called either a zygote or an embryo, depending upon its stage of development -- depends upon the uterine lining for nutrients and oxygen until about the fourth month of gestation. By this time, the embryo -- now called a fetus -- has produced the placenta, which provides nutrition and oxygen. Until placental formation is complete, however, the developing baby must continue to secrete hCG to maintain the corpus luteum, which in turn maintains the uterine lining, explains Dr. Sherwood.
Pregnancy Tests
Your hCG levels provide your obstetrician with information about your pregnancy, explains Dr. Miriam Stoppard in her book "Conception, Pregnancy and Birth." Home pregnancy tests -- and urine-based pregnancy tests that you take at a doctor's office -- detect hCG in your urine. Since only pregnant women have hCG in the urine, detection of any of the hormone indicates pregnancy. Urine tests can't tell how much hCG you have in your urine; they simply indicate presence or absence.
HCG Levels
Your doctor can also test your blood for quantitative hCG level. This test helps provide information about how your pregnancy is progressing, note Heidi Murkoff and Sharon Mazel in their book "What To Expect When You're Expecting." In a normal pregnancy, hCG levels double every 48-72 hours through early pregnancy. Low hCG levels, or hCG levels that don't rise as expected, can indicate an ectopic -- or tubal -- pregnancy, or something else abnormal.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Conception, Pregnancy and Birth"; Miriam Stoppard, M.D.; 2008
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
