Early in pregnancy, your developing embryo begins to secrete a hormone called human chorionic gonadotropin, or hCG. This hormone helps to communicate to your body that you're pregnant, and helps to maintain the pregnancy. Your doctor can use your hCG levels to get information about the health and viability of your pregnancy.
HCG
Each month after you ovulate, your uterine lining begins to thicken in response to rising levels of estrogen and progesterone from ovarian tissue called the corpus luteum. Normally, the corpus luteum disintegrates after about 14 days, and hormone levels fall off. This causes you to menstruate. If you become pregnant, you need to maintain the uterine lining. HCG from the developing embryo maintains the corpus luteum, which in turn maintains high estrogen and progesterone levels, preventing menstruation.
Pregnancy Testing
Because hCG doesn't come from you -- it comes from your developing baby -- you'll only have hCG in your bloodstream and urine if you're pregnant. Home pregnancy tests check for hCG in the urine, which typically reaches levels that a test can detect around the time of your missed period. Your doctor also uses hCG to confirm pregnancy -- they may test your urine or your blood.
HCG Levels
Your obstetrician can use hCG as more than simply an indicator of pregnancy -- they can also do a quantitative hCG test to see how much of the hormone you have in your body. This can provide some information about how far along you are because hCG levels generally fall into ranges that correlate to the age of the pregnancy. HCG levels tend to rise until the end of the first trimester, and then fall slightly.
Abnormalities
Your hCG levels in early pregnancy can also give your doctor information about pregnancy viability. HCG should rise predictably in the early weeks and months of pregnancy, doubling every 48 to 72 hours. HCG levels that don't rise in this way may indicate an ectopic pregnancy, which is one in which the developing egg has implanted outside the uterus. Ectopic pregnancies, which aren't viable, typically show low hCG levels and are slow to double.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- American Pregnancy Association: Human Chorionic Gonadotropin (hCG): The Pregnancy Hormone
- Advanced Fertility Center of Chicago: Ectopic Pregnancy -- Tubal Pregnancy
