Can a Pregnancy Test Be Wrong?

How Reliable Are Home Pregnancy Tests, Really?

When home pregnancy tests became available in the late 1970s, they revolutionized the way potential parents-to-be discovered the momentous news of a confirmed pregnancy. Previous generations had to just wait and see, but today’s women enjoy the enormous benefit of widely available, inexpensive, easy-to-use and accurate home pregnancy tests. Used correctly and at the right time, pregnancy tests are almost 100 percent reliable. If the result says you are pregnant—believe it! Negative results, on the other hand, are not always what they appear, but repeating the test should clear up the mystery.

Accuracy of Pregnancy Tests

Looking at pregnancy test result

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When used correctly, home pregnancy tests are about 99 percent accurate. Using home tests correctly means making sure the test is not expired or damaged, following the instructions diligently, and taking the test at the right time. The latter is key in preventing false negative results. Home pregnancy tests detect the presence of hCG (human chorionic gonadotropin), the hormone present in a woman’s urine when she is pregnant. The hormone is released into the bloodstream and urine after a fertilized egg implants in the uterus. Its concentration increases rapidly, doubling every 48 hours after implantation. Different brands of home pregnancy tests can detect hCG in the urine at different concentrations, which is why some claim to provide reliable results on the first day of, or even days before, a missed period, while others require that you wait up to a week after you miss a period.

Tips for improving the accuracy of home pregnancy test results include:

  • Pay attention to the time frame given in the test’s instructions for reading the results. Look too soon or wait too long after taking the test, and the results might be incorrect. Use a watch instead of counting seconds or guessing how much time has elapsed. 
  • Take the test first thing in the morning after at least four hours without urinating. “First morning urine” has a higher concentration of hCG than urine passed later in the day.
  • If you have difficulty reading the results of a standard test, opt for a digital version, which displays the words “pregnant” or “not pregnant” instead of a series of lines. 
  • If you have any doubts that you took a test correctly or any reason to suspect incorrect results, simply take another test. You also might opt to take another test at your doctor’s office.

False Positives

True false positive results on a home pregnancy test are very rare. If a home pregnancy test says you are pregnant, then congratulations – you are. Begin routine pregnancy care immediately and call your doctor to book your first prenatal appointment.

The only exceptions (if the test was taken correctly) are for women taking certain medications and those with a few unusual medical conditions. In these cases, your doctor should make you aware of the possibility of false positive pregnancy test results.

Another explanation for what appears to be a false positive pregnancy test is a missed miscarriage. A woman might be pregnant and get a positive result at the time of taking the test, but soon after experience a very early miscarriage. The timing and symptoms of such a loss can closely resemble menstruation.

False Negatives

Looking at pregnancy test result

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False negative pregnancy test results are a real possibility, especially if you took the test earlier than a week after your missed period. If you ovulated later than expected, implantation and the associated release of hCG could come later in your cycle than expected. This means it might take longer than average to get accurate results from a test.

Re-read the test’s instructions and determine whether you have are long enough into your cycle to get accurate results from that particular brand of test. Also consider whether or not your period really is late. Many women have irregular cycles, and it’s not unusual for some fluctuation in cycle length, even among women who usually have regular periods.

If you get a negative result but believe you could still be pregnant, wait at least a few days or up to a week, and if your period doesn’t come, test again. Repeated negative test results in the absence of your period indicate another reason for the missed period, so call your doctor for an appointment.