Human chorionic gonadotropin, or hCG, is an important pregnancy hormone that helps signal to your body that you're pregnant and helps you maintain the pregnancy. Falling levels of hCG during pregnancy, depending upon when they occur, can signal that something is wrong. Alternately, they can be totally normal and natural.
Your Body
During your normal monthly reproductive cycle, your ovaries ripen and release a mature egg. Specialized tissue called the corpus luteum, located in the ovary that released the egg, then begins to release estrogen and progesterone. Rising hormone levels cause the lining of your uterus to thicken, says Dr. Lauralee Sherwood in her book "Human Physiology." This gives the egg a place to implant, if it's fertilized. If it's not fertilized, the corpus luteum dies after about 14 days, and you menstruate.
Human Chorionic Gonadotropin
If conception occurs, the fertilized egg has to signal your body that fertilization has taken place to prevent menstruation and loss of the uterine lining. The egg does this by secreting hCG starting quite soon after fertilization. HCG maintains the corpus luteum, which in turn maintains the lining of the uterus. HCG production is vital to maintenance of pregnancy until the placenta has fully developed, around three or four months of gestation.
HCG Levels
Because only pregnant women -- and those taking fertility drugs -- have hCG in their bloodstreams, it's possible to test for hCG in the blood or in the urine to test for pregnancy. This, explain Heidi Murkoff and Sharon Mazel in their book "What To Expect When You're Expecting," is the basis of a home pregnancy test. HCG should double every 48 to 72 hours in early pregnancy, so your obstetrician can test your blood for quantitative hCG levels to assess your pregnancy progress.
Falling HCG
Through about the third month of pregnancy, your hCG levels should double regularly and should be above certain threshold values. The Advanced Fertility Center of Chicago notes that low hCG can indicate an ectopic pregnancy, while falling hCG levels may be one of the first indications that a pregnancy is not viable and will spontaneously terminate. There is very little your doctor can do to prevent a miscarriage associated with falling hCG early in pregnancy.
Later In Pregnancy
Once you've reached about the third or fourth month of pregnancy, your fetus is no longer as dependent upon the lining of the uterus. Instead, it has built a placenta that provides for its nutritional needs. The placenta produces progesterone, which maintains the uterine lining, and replaces the corpus luteum. Because you no longer need the corpus luteum, the fetus stops secreting as much hCG, and hCG levels gradually drop through the remainder of pregnancy, says AmericanPregnancy.org.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
- Advanced Fertility Center of Chicago: Hormone Levels
- AmericanPregnancy.org: HCG Levels
