Pregnancy Test Facts
*Pregnancy test facts medically edited by: Charles Patrick Davis, MD, PhD
- Pregnancy tests work by detecting a hormone in a woman's blood or urine: hCG (human chorionic gonadotropin) is produced when an egg implanted in the uterus.
- There are two types of pregnancy tests, one uses blood and the other uses urine (home tests); a quantitative blood test detects hCG earlier than a qualitative blood or urine test.
- Most home pregnancy tests detect hCG in a urine stream when, after a few minutes, a line or symbol appears if the test is positive (indicating pregnancy); most urine pregnancy test instructions urge a second test in a few days no matter what were the first results; follow package instructions – if a control indicator does not appear, the test result should not be trusted.
- Home tests are accurate, some better (more sensitive in showing early pregnancy) than others.
- Home pregnancy tests may test positive on the first day of a missed period, but almost all detect pregnancy 1 week after a missed period.
- Call your doctor for an appointment if you test positive for pregnancy with a home test.
- It is possible to be pregnant and have a negative home pregnancy test; that is why you should wait about a week to repeat the test. Most medicines and other compounds (alcohol, drugs) do not interfere with pregnancy tests; however, any drug containing hCG (some fertility drugs) may give false test results.
- Any questions or problems with pregnancy tests should be discussed with your doctor.
How do pregnancy tests work?
All pregnancy tests work by detecting a certain hormone in the urine or blood that is only there when a woman is pregnant. This hormone is called human chorionic gonadotropin (kohr-ee-ON-ihk goh-NAD-uh-TROH-puhn), or hCG. It is also called the pregnancy hormone.
hCG is made when a fertilized egg implants in the uterus. This usually happens about six days after the egg and sperm merge. But studies show that in up to 10 percent of women, implantation does not occur until much later, after the first day of the missed period. The amount of hCG rapidly builds up in your body with each passing day you are pregnant.