The hormone hCG, or human chorionic gonadotropin, is a hormone that your developing baby secretes to communicate to your body that you're pregnant. HCG has critical roles in early pregnancy--through around the third or fourth month--but isn't as important to late pregnancy. Your hCG levels typically rise through the early portion of your pregnancy, and drop later.
Female Reproductive System
The normal female reproductive cycle doesn't include production or release of hCG--in fact, adults can't produce hCG at all. When you ovulate each month, tissue called the corpus luteum--a group of cells located in the ovary--secretes estrogen and progesterone. This thickens the uterine lining in preparation for fertilization of an egg. If you don't conceive, the corpus luteum dies after about two weeks, and you'll menstruate.
Role of hCG
Assuming that you conceive, it's an early job of the fertilized egg to let your body know you're pregnant, so that your uterine lining remains intact. The developing egg does this by releasing hCG from cells that will eventually become the placenta and membranes surrounding the baby. HCG keeps the corpus luteum functional, explains Dr. Lauralee Sherwood in her book "Human Physiology," which in turn maintains the uterine lining.
HCG Levels
Your hCG levels rise during early pregnancy, explains the American Pregnancy Association, doubling every 48 to 72 hours. Pregnancy tests, whether at home or in the doctor's office, check the urine for hCG. Until such time as the placenta develops, your growing baby is dependent upon hCG to maintain the corpus luteum, which in turn maintains the lining of the uterus by continuing to secrete the hormones estrogen and progesterone.
Late Pregnancy
After around three or four months gestational age, the placenta is fully developed. This organ provides your fetus with nutrients and oxygen that it extracts from your bloodstream, and also produces progesterone, which maintains the uterine lining. The corpus luteum is no longer necessary at this point, and hCG levels start to fall. As they do so, the corpus luteum gradually disintegrates. Your hCG levels remain measurable but much lower through the remainder of your pregnancy.
Expert Insight
While falling hCG in early pregnancy indicates a problem with the pregnancy--for instance, it can indicate a tubal pregnancy or a threatened miscarriage--falling hCG in late pregnancy is completely normal. Once the placenta takes over progesterone production and the corpus luteum becomes irrelevant to maintaining the pregnancy, fetal tissue stops producing significant quantities of hCG--though your baby's chorionic membranes will make some of the hormone throughout your pregnancy.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- American Pregnancy Association: Human Chorionic Gonadotropin (hCG): The Pregnancy Hormone
