When you conceive, the first task of the fertilized egg is to signal your body that you've become pregnant. This prepares your body to support and maintain the pregnancy, and prevents menstruation. The egg communicates with your body using hCG, or human chorionic gonadotropin, a hormone that increases in concentration through the third or fourth month of pregnancy.
Your Menstrual Cycle
Your ovaries ripen an egg, and then ovulate that egg, each month. Once the egg leaves the ovary, leftover tissue from the egg's follicle in the ovary starts secreting the hormones estrogen and progesterone. This tissue is called the corpus luteum, and it's directly responsible for the fact that your uterine lining thickens, since the thickening occurs in response to the hormones. After about two weeks, the corpus luteum dies, and your hormone levels fall. This causes you to menstruate.
Fertilization
If the released egg is fertilized, it's quite important that your uterine lining remain in place, since the lining of the uterus supports the developing embryo for the first several months of pregnancy. As such, the fertilized egg, as it travels through the fallopian tube and begins to divide, secretes hCG to communicate with the corpus luteum. The hCG causes the corpus luteum to remain in place, and it continues to secrete estrogen and progesterone, maintaining the lining of the uterus.
HCG Levels
Adult tissue doesn't secrete hCG--this hormone comes only from specialized areas of a developing embryo. As such, unless you're taking fertility drugs, which can contain hCG or hCG-like substances, there's no way for you to have hCG in your system unless you're pregnant. Home pregnancy tests--and your doctor's office--use hCG presence or absence to determine whether you're pregnant, explain Heidi Murkoff and Sharon Mazel in their book "What To Expect When You're Expecting."
Level Increase
In a normal pregnancy, hCG levels start rising significantly within just a few days of conception. By about one week after conception, or about a week before your missed period, you typically have enough hCG in your bloodstream--and therefore in your urine--for sensitive pregnancy tests to detect it. HCG should double every 48 to 72 hours, explains the American Pregnancy Association. This occurs until around the third or fourth month of pregnancy, at which point hCG levels start to fall through the remainder of your nine months.
Pregnancy Problems
The Advanced Fertility Center of Chicago notes that if there is a problem with your pregnancy, often your physician can tell early on through abnormal hCG levels. Ectopic, or tubal, pregnancies often don't result in the same high levels of hCG as normal uterine pregnancies. Further, in ectopic pregnancies, hCG levels rise slowly. Your doctor's office may perform a quantitative hCG level test, using your blood, at several different points in your early pregnancy to assess the viability of the pregnancy.
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
