Any type of pregnancy test, including urine and blood, work by measuring a specific hormone in a woman's body that is only present during pregnancy. WomensHealth.gov, the U.S. Department of Health and Human Services website, calls this hormone the pregnancy hormone, or human chorionic gonadotropin. The body begins making hCG when the fertilized egg attaches to the uterine lining, about six to 10 days after conception. The more days that pass the greater the hCG builds up in the body.
Presence of hCG
The two types of blood tests are a qualitative and a quantitive measurement. Both tests identify whether hCG is present in a woman's body. Womenshealth.gov explains that the qualitative blood test is similar to a urine test in that it only reveals whether the hormone is present and nothing more.
Quantity of hCG
The quantitative blood test, also called beta hCG test, identifies the exact amount of the hormone in the blood, explains Womenshealth.gov. It is very accurate and capable of detecting even a minute amount of the hormone.
Age of Pregnancy
With the quantitative blood test a medical care provider can estimate about how long a woman has been pregnant. hCG tends to double every 72 hours, as indicated by the American Pregnancy Association. As the pregnancy progresses, the time to double increases to about 96 hours. Estimating the age of the pregnancy is more accurate using an ultrasound after the fifth or sixth week of gestation.
