By Elaine Brown, MD
Manufacturers of today's home pregnancy tests claim that they are 99% effective when properly used. The only more accurate tests are blood tests administered by physicians. But when and how to use them, and which are the most accurate are questions that still remain. Are they all the same? What if the test is negative, but "I just have that feeling…"?
Pregnancy test work by detecting beta-hCG (human chorionic gonadotropin), a hormone produced by the placenta in a pregnant woman's uterus. The hormone is released into the blood stream and ultimately excreted in the urine. HCG usually becomes detectable in the urine about 10 days after conception, or about 4 to 5 days before the next missed menstrual period. Because it can be difficult to know exactly when conception occurred, or when exactly your next period is supposed to start, most experts recommend waiting 5 to 10 days after your period is late to test for pregnancy; this helps ensure that the test is accurate (and can help save you money by not testing yourself multiple times).
Most pregnancy tests on the market will show a reliable result by 7 to 10 days after the missed menstrual period. Often though, it may be desirable to have the result earlier. Some of the home tests are accurate enough to detect the HCG as soon as it is present, even at very low levels. Two studies have confirmed claims of the manufacturers of First Response™ Early Result Pregnancy Test to detect a pregnancy 5 days before the missed period. Clearblue® claims to be accurate within 4 days before the missed period. If you want to know as quickly as possible, these two brands are probably the best tests to use, but you will need to be willing to pay a little extra for that privilege!
It is certainly possible to have a false negative (a test result that is negative when you are actually pregnant). A study done in 2004 evaluated 18 different home pregnancy tests and found that they missed up to 85% of pregnancies when testing was done on the first day of the missed period. Almost all of them returned positive results by one week after the missed period. The moral of the story: If your first test is negative, wait a week and test again.
Here are a few things you can do to improve the accuracy of your home pregnancy test:
Other things to note: A pregnancy problem (impending miscarriage, tubal pregnancy, etc.) can cause a false negative result. Rarely infertility medications can cause a false positive result (test result is positive when the woman is not pregnant). Finally, HCG causes some of the typical early pregnancy symptoms such as breast tenderness and nausea, so eventually your own body will be likely to alert you.
The best way to double check if you suspect your test is not accurate, or if you simply need peace of mind, is to visit your doctor or local clinic. Many doctor's offices or clinics offer free pregnancy tests, so inquire before going in.
For more information, see the Pregnancy Tests Fact Sheet, provided by the Office on Women’s Health.
Dr. Elaine Brown completed her residency in obstetrics and gynecology at Harvard. She has more than 15 years of experience in private practice.
Published March 6, 2014