Beta human chorionic gonadotropin (hCG) testing refers to a blood test that can read the exact levels of hCG in a woman's blood. This can also be called a quantitative hCG test, and it is the most accurate test available for detecting pregnancy in its earliest stages. By comparison, urine pregnancy tests and and qualitative blood tests only detect the presence of hCG, but they are generally not sensitive enough to read levels below a certain amount. Beta hCG levels are therefore highly recommended for verifying a pregnancy as early as possible, and for checking on the health of a known pregnancy by ensuring that the readings of hCG are normal.
HCG is a hormone produced by cells which will become the placenta and is needed to alert the body that a pregnancy has occurred. This tells the brain and ovaries not to secrete additional hormones to bring about a monthly menstrual period, but to instead begin supplying hormones to support the pregnancy and its needs. Beta hCG levels should be able to predict a pregnancy within a day or two of implantation, which generally occurs four to seven days after conception. Implantation is when the group of cells which will become the baby, placenta, and yolk sac embed into the lining of the uterus.
The exact beta hCG levels will vary widely from woman to woman and from pregnancy to pregnancy. For this reason, hCG levels should not be used to determine the gestational age of the embryo or to determine a pregnancy's viability unless being compared to previous results. In most cases, as long as beta hCG levels are going up and doubling approximately every 48 hours, the pregnancy is considered viable. If levels go down or stay the same, this could indicate an early miscarriage.
Doctors do not normally perform routine tests to check beta hCG levels in detected pregnancies unless a problem is suspected. Issues that warrant testing may include vaginal bleeding, severe cramping, or a past miscarriage in the mother. Tests may be taken to get hCG levels and then taken again two days later to ensure that the hormone levels are increasing at the appropriate rate. It should be noted, however, that in some cases hCG levels rise more quickly or slowly, even in healthy pregnancies. If beta hCG levels are rising at a slower than average rate, a trans-vaginal ultrasound may be the best way to determine whether or not the pregnancy is viable.