When a woman becomes pregnant, her body begins to produce the hormone known as human chorionic gonadotropin (HCG) to form and maintain a placenta around the developing egg. A qualitative blood or urine test merely measures if HCG is present and is the common method of testing for pregnancy. A quantitative blood test, by contrast, will measure the levels at which HCG appears in order to determine the approximate due date and whether any abnormalities or miscarriage might occur.
HCG levels increase according to a fairly predictable time table until women are about five months pregnant, and then that level maintains until birth. According to the American Pregnancy Association, about 85 percent of pregnant women will have HCG levels that double every two or three days, until the end of the first trimester, when those levels start to take about four days to double. Since these numbers are fairly predictable, doctors can order a quantitative blood test to measure the probability of a variety of problems.
The National Institutes of Health maintains a list of appropriate ranges of quantitative blood test levels, based on the range of weeks a woman has been pregnant. If the test reveals HCG levels that are too low, it could mean miscarriage, partial miscarriage or ectopic pregnancy, which means the fetus is developing outside the uterus. Another set of potential problems, or surprises, could be signaled if the levels are too high, from cancer and internal moles to twins or more than two fetuses developing concurrently.
A quantitative blood test is used in concert with other tests if Down Syndrome is suspected. It is also regularly used to determine a woman's due date before an ultrasound can more accurately determine this date. This test may go by several names, depending on the lab and doctor prescribing it. Serial beta HCG and repeat quantitative beta HCG are other common names for this test.
If a quantitative blood test reveals too high or too low HCG levels, doctors will start to hone in on the particular problem. Medical imaging and further blood tests might be ordered to identify the specific condition a patient is experiencing. Follow-up testing, particularly an ultrasound at five or six weeks, might reveal that no problems exist, since some women have abnormal HCG levels and give birth to healthy children.