Your body produces a number of hormones each month in the course of a normal menstrual cycle. These hormones lead to the changes you experience as your body prepares for the fertilization and implantation of an egg. HCG, which can come from a fertilized egg, a tumor or certain fertility treatments, can alter the menstrual cycle.
Hormone Production
Your normal monthly reproductive cycle begins on the first day of your period. During this time, as you shed the lining of the uterus that thickened during your last cycle, your ovaries begin to ripen an egg for ovulation. The tissue around the egg secretes the hormones progesterone and estrogen. These hormones, as they build up in the body, cause the lining of the uterus to begin thickening once again, which begins around day seven of your menstrual cycle, about seven days before ovulation.
Ovulation and Corpus Luteum
Around day 14 of your menstrual cycle, high hormone levels cause the ovary to release an egg. Tissue from around the egg remains in the ovary and continues to produce high levels of estrogen and progesterone -- this tissue is the corpus luteum. If the egg isn't fertilized, the corpus luteum spontaneously deteriorates after around 14 days, leading to menstruation, or shedding of the uterine lining. This represents the completion of a normal menstrual cycle.
HCG Production
Your body doesn't actually produce hCG; instead, the hormone comes from tissue in a fertilized egg. If, after you ovulate, the mature egg is fertilized by a sperm, it's up to the egg to communicate to your body that you're pregnant. This maintains the corpus luteum, which keeps producing estrogen and progesterone, and maintains the uterine lining. The fertilized egg needs the uterine lining in place in order to have somewhere to implant, explains Dr. Lauralee Sherwood in her book "Human Physiology."
Alternately, some fertility drugs contain hCG -- Pregyl is an example. Such drugs general result in a cessation of menstruation, but increase the likelihood of ovulation, which is why they're used to treat certain kinds of infertility.
HCG Effects
There are several effects of hCG upon female physiology. The first is that it results in cessation of the menstrual cycle; for as long as you're pregnant and have high levels of hCG in your blood, you won't be shedding the lining of the uterus, and won't have a period. Another effect is that hCG seems to cause nausea, leading to the morning sickness so common to early pregnancy, explain Heidi Murkoff and Sharon Mazel in their book "What To Expect When You're Expecting."
HCG Testing
Because only pregnant women, those with tumors -- though this is rare -- and those being treated with certain fertility drugs, have hCG in their blood, it's possible to test for pregnancy by checking for the presence of hCG. Your doctor may use either a blood test or a urine test; home pregnancy tests always check for the hormone in your urine. Typically, hCG levels reach measurable quantities before you even miss a period, note Murkoff and Mazel -- often by as much as a week before your missed period.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
