HCG, or human chorionic gonadotropin, is an important hormone that helps let your body know you're pregnant. This prevents menstruation, and maintains the lining of the uterus, which nourishes your embryo and fetus until the placenta finishes developing. A drop in hCG can signal that something is wrong with a pregnancy.
The Uterine Lining
Normally, your uterine lining proliferates and then sloughs off once each month. Hormones regulate this process--when you ovulate, tissue called the corpus luteum that forms in your ovary produces estrogen and progesterone, which causes the lining to proliferate. If you don't conceive a child, the corpus luteum eventually degrades, and the lining of the uterus sloughs. This results in menstruation, or the loss of blood and cells from the uterine lining.
Importance of hCG
The purpose of hCG is to communicate with the corpus luteum and keep it alive and functioning, which keeps levels of estrogen and progesterone high. In turn, the estrogen and progesterone maintain your uterine lining, which is where a fertilized egg implants. Until the placenta develops--around the third or fourth month of pregnancy--your embryo will be entirely dependent upon the uterine lining for nutrients and oxygen.
Your physician can use your hCG levels to assess the presence or absence--and to a certain extent the viability--of your pregnancy. Pregnancy tests at the doctor's office--like home pregnancy tests--check for the presence of hCG. Explain Heidi Murkoff and Sharon Mazel in their book "What To Expect When You're Expecting," since only pregnant women have hCG in their urine, positive hCG indicates pregnancy.
HCG levels should rise predictably, explains the American Pregnancy Association. If they don't double every 48 to 72 hours, this can indicate that something is wrong with the pregnancy. Further, if hCG levels fall prior to the third or fourth month of pregnancy, the pregnancy may be headed for miscarriage. There are many reasons pregnancies self-terminate early on; most of them relate to chromosomal abnormalities, and can't be prevented by any medical means. There is nothing your doctor can do to save your pregnancy if your hCG levels start to drop of their own accord.
Normal HCG Drop
Around the third or fourth month of pregnancy, hCG levels start to drop on their own. This is because your fetus has a fully formed placenta, which brings oxygen and nutrients to the baby through the umbilical cord. The placenta secretes progesterone and other hormones, maintaining the lining of the uterus and rendering the corpus luteum unnecessary. The corpus luteum degrades, but because the placenta has taken over hormone production, you maintain your pregnancy despite falling hCG levels.