Pregnancy Tests
How do pregnancy tests work?
Pregnancy tests detect a hormone in the urine or blood called human chorionic gonadotropin (hCG).
Blood tests can detect hCG earlier in a pregnancy than urine tests can. Ovulation occurs when an egg is released from the ovary approximately 2 weeks after the first day of the last menstrual period (LMP).
Blood tests can detect if you are pregnant about 1 week after ovulation or about 3 weeks from your LMP.
Urine tests can detect a pregnancy about 2 weeks after ovulation or as early as one day after you miss a menstrual period.
How accurate are home pregnancy tests?
Urine home pregnancy tests (HPTs) are very accurate, up to 97% to 99%, if a test is done correctly. Check the package for an expiration date before using it.
Most HPTs will be accurate if you test yourself around the time of your period. You can get a negative HPT if you use the test too early in your pregnancy or if you ovulated later than you thought. You can repeat a test again in a few days no matter what the results.
Can anything interfere with HPT results?
Most medications, both over-the-counter and prescription drugs, including birth control pills and antibiotics, do not affect the results of a home pregnancy test.
July 2006
