Your body begins producing hCG, or human chorionic gonadotropin, soon after conception, according to Dr. Marjorie Greenfield in an article for "Dr. Spock" magazine. Created by the placenta, hCG lets the rest of your reproductive system know that you're pregnant so it can change its normal routine. Rather than sloughing off the uterine wall by menstruation, your body will increase progesterone production instead, according to Greenfield. This has the opposite effect of maintaining your uterine wall to sustain your pregnancy.
Measurements
Greenfield says that hCG is detectable in your blood even before your period is due. Three weeks after your last menstrual period, your hCG level should range from 5 to 50 mIU/ml, or milli-international units per milliliter. That level should begin doubling every two to three days through your first trimester. It then begins dropping, according to the American Pregnancy Association. Therefore, it is normal for your hCG level to be in the area of 280,000 mIU/ml three months after your last period, for example, and lower to 250,000 mIU/ml by the next time you see your doctor, according to the website JustMommies.
Testing
Blood tests are more sensitive than urine tests for hCG detection. According to the American Pregnancy Association, there are two kinds of blood tests to measure hCG level. A qualitative test simply makes sure that HCG is present. A quantitative test tells your doctor what your level is so he can monitor whether or not it is doubling as it should. Greenfield says this is useful not only to ensure that your pregnancy is progressing properly, but to give your doctor an idea of what he should expect to see on an ultrasound. If your HCG level is lower than 2,000 mIU/ml, your baby probably will not be visible on a transvaginal ultrasound.
Implications
Greenfield says that if your hCG level stops increasing during your first trimester or actually declines, this generally means that your pregnancy isn't viable and won't continue to term. This is also true if your level increases, but only by half or less.
Causes
If your hCG level is on the low end of the spectrum of normal for your stage of pregnancy--as opposed to actually decreasing--you might have simply miscalculated the date you think you got pregnant. But if it is decreasing, you might have a tubal or ectopic pregnancy or other complications that could result in miscarriage.
Considerations
The Advanced Fertility Center of Chicago advises that low hCG levels do not automatically mean there is a problem with your pregnancy. The American Pregnancy Association notes that as long as your level is doubling along predictable time patterns, it doesn't matter how low it is. There is a wide range of normal and it is different for each woman. Though your doctor would ideally like to see your level double, Greenfield says that the lower end of the spectrum for an increase is approximately 66 percent over a test done two days before.
