When a fertilized egg begins to divide into a cell ball called a zygote, it communicates to your body that you're pregnant by using the hormone hCG, or human chorionic gonadotropin. If your hCG levels don't increase appropriately, it can be a sign that something is amiss with the pregnancy.
Reproductive Cycle
Despite the fact that hCG is critical to a healthy pregnancy, it's not a hormonal component of your normal monthly reproductive cycle. Each month, your estrogen and progesterone levels rise as tissue in the ovary, called the corpus luteum, secretes these hormones. In the absence of a pregnancy signal from a fertilized egg, the corpus luteum secretes hormones for only about 14 days, at which point it regresses and hormone production falls. This leads to sloughing of the uterine lining.
Role of HCG
Since the uterine lining is where a fertilized egg implants when it reaches the uterus, it's crucial that the corpus luteum remain active and continue secreting estrogen and progesterone, which maintain the lining. Shortly after fertilization, a zygote begins secreting hCG, which maintains the corpus luteum. This in turn results in maintenance of the uterine lining and the absence of menstruation.
HCG Levels
Unless you're pregnant, you don't have hCG in your blood or urine. Once a zygote begins secreting hCG, however, the hormone levels quickly rise. Home pregnancy tests can detect hCG in the urine as soon as a week after conception, though many are more accurate around the time of your missed period, around two weeks after conception. Heidi Murkoff and Sharon Mazel explain in their book "What To Expect When You're Expecting" that hCG should double every 48 to 72 hours in early pregnancy.
Low HCG Levels
If hCG levels are low, there are several possible explanations. The first is that the pregnancy might not be as far along as you originally thought. Your doctor will test for hCG twice in order to determine whether the low levels are caused by an abnormality, or to a younger pregnancy than suspected. As long as levels double appropriately, the pregnancy is normal, explain Murkoff and Mazel.
Abnormal Pregnancy
If your hCG is low and doubles slowly, you may have an ectopic pregnancy, or tubal pregnancy, explains the Advanced Fertility Institute of Chicago. This kind of pregnancy occurs when an egg implants outside the uterus. It's not a sustainable pregnancy and generally shows low and slow-to-double hCG in addition to other symptoms. Some embryos implant in the uterus but fail to develop normally, note Murkoff and Mazel. This is typically unavoidable and is caused by chromosomal abnormalities, and may also result in low hCG. Such pregnancies end via miscarriage.
References
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
- Advanced Fertility Center of Chicago: hCG
