A pregnancy test can tell whether you are pregnant by checking for a particular hormone in your urine or blood. The hormone is called human chorionic gonadotropin (HCG). HCG is made in a woman's placenta after a fertilized egg implants in the uterus. It is normally made only during pregnancy.
A pregnancy blood test is done in a health care provider's office. It can find smaller amounts of HCG, and can confirm or rule out a pregnancy earlier than a urine test. A blood test can detect pregnancy even before you've missed a period. Pregnancy blood tests are about 99 percent accurate. A blood test is often used to confirm the results of a home pregnancy test.