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When to take a home pregnancy test and how they work

Updated on November 23, 2015.

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 tests work by detecting beta-hCG (human chorionic gonadotropin), a hormone produced by the placenta in a pregnant woman's uterus. This hormone is released into the blood stream and ultimately excreted in the urine. It 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's 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 6 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's 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:

• Use urine collected first thing in the morning — it's more concentrated. You can collect the urine and save it in a labeled container in the refrigerator if you need to run to the store for the test kit.
• Don't drink too much fluid before testing. This will dilute the urine.
• If you are testing later in the day, make sure it has been at least 4 hours since you last emptied your bladder.
• Check the expiration date on your test.
• Follow the directions exactly as specified on the package insert.
• Read the instructions to make sure you are not taking any medications that will alter the test results.

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.

Published on March 6, 2014. 

Dr. Elaine Brown completed her residency in obstetrics and gynecology at Harvard. She has more than 15 years of experience in private practice.