The most common error with home pregnancy tests is that they are done too early. A pregnancy test detects the presence of human chorionic gonadotropin, or hCG. This hormone is secreted by a fertilized egg and is usually detectable shortly after implantation occurs (usually around 21 days after a woman’s last menstrual period).
If done too early, a urine pregnancy test will give a negative result when the woman is actually pregnant because the hCG level is too low to detect. This is why a woman who suspects she is pregnant, though her test is negative, should re-test shortly after her missed period was due.
Other reasons a pregnancy test may give a negative result when a woman is actually pregnant exist as well: the test was read too soon, the specimen was mixed up with another patient (if done in an emergency room and not at home, for example), the hCG level was extremely high (which can affect antigen binding and make the test not work), the test reader was color blind, or severe kidney disease could interfere with the hCG level detection.
Positive pregnancy tests can be false, too. Recent injections with hCG, which are part of some fertility treatments, can lead to a falsely positive pregnancy test. Certain herbal medications can do the same. Some women may have circulating hCG from a tumor that secretes this hormone and are not pregnant at all (thankfully, this is not common!).
Another interesting scenario is when a woman has a positive blood pregnancy test but a negative urine pregnancy test, and she is not actually pregnant. This is often due to circulating heterophilic antibodies, which mimic the hCG hormone and cause the blood pregnancy test to look positive. These antibodies are often found in women who work around animals, since the heterophilic antibody has reacted to animal antigens. These antigens are used in the blood pregnancy test, thus resulting in a positive test!
Therefore, if a urine test is negative and confirmation is needed, a blood pregnancy test should be done. If results are still not clear, your provider may order a repeat blood test in a few days, with or without an ultrasound, to better understand your test results. However, if an ultrasound is done too soon, it may miss a pregnancy that is too small to be seen, so it may need to be repeated as well.
Reviewed by Dr. Jen Lincoln, December 2018
- Pregnancy tests are not perfect and can be falsely negative and positive.
- The most common reason a home pregnancy test is “wrong” is because it was done too soon.
- Certain treatments and medications can make pregnancy tests positive when you are not really pregnant.