How Do Pregnancy Tests Work?

By Chris Sherwood


There are two different types of pregnancy tests. The first, and most accurate test, is done in a doctor's office and uses a blood sample to test for a pregnancy hormone called human chorionic gonadotropin (hCG). The second and most commonly used pregnancy test is the home pregnancy test (HPT). This method can be done at home and uses urine to detect hCG.

Human Chorionic Gonadotropin

When a fertilized egg successfully implants itself in the uterus, the body produces a hormone called human chorionic gonadotropin (hCG). The hormone is usually produced six days after the sperm and egg unite, but can take all the way up until the first period does not occur. The longer the fertilized egg has been implanted in the uterus, the greater amount of the hCG hormone will be present in the body.

Home Pregnancy Test

A home pregnancy test (HPT) is done by placing a small amount of urine on a test stick. If there is hCG present in the urine sample, the chemical reaction on the test stick will cause a positive result to be displayed. If no hCG is present in the sample, the test stick will display a negative result. Each test stick has different directions on how long you will need to wait before a test result will be displayed.

Doctor Pregnancy Test

Home pregnancy tests are convenient and inexpensive, but if taken too early, the test may display a false negative. This is because hCG can be detected in the blood much sooner than in the urine. Doctor's can use one of two blood tests to determine pregnancy. The first test is quantitative. This test measures exactly how much hCG hormone is present in the blood sample. The second test is qualitative and only detects whether or not hCG is present, not how much of the hormone is in the blood sample.