How Often Are Pregnancy Tests Wrong?

By April Sanders
An old, expired test might give a false negative.
An old, expired test might give a false negative.

Pregnancy tests are usually about 97 percent accurate, according to WebMD. But that figure, which reflects the accuracy of urine-based home pregnancy tests, is dependent on several factors, including the timing of the test and whether it was taken correctly. Such factors can cause the test to produce a false negative, and more rarely, a false positive.

Testing Too Early

Timing is everything when it comes to taking a home pregnancy test. Waiting until a week after your missed period will greatly increase your chances of getting an accurate result. If you take the test too early -- say before you even miss your period -- you run the risk of a false negative.

Testing Too Late

Get the most accurate results by taking a pregnancy test early in the morning. Right after you wake up, your urine has the highest concentrate of hCG, or human chorionic gonadotropin, it will have all day. This hormone is produced by the placenta directly after egg implantation and ends up in your urine. Taking a pregnancy test early in the morning, when your urine is concentrated, gives you the most accurate results. Taking it later in the day or after you've had a lot of water -- thus diluting your urine -- may give you a false negative.

Ignoring the Directions

It might not seem difficult to pee on a stick, but if you don't follow the directions carefully, you might end up with incorrect results. Wait the exact allotted amount of minutes before looking at the results. If you take an early peek, the test might return a "not pregnant" result when you are really pregnant.

Using Certain Drugs

The use of fertility drugs that contain hCG can lead to false positives on pregnancy tests, according to The Mayo Clinic. Other drugs besides fertility drugs can mimic the effect of hCG on pregnancy tests, including some hypnotics, diuretics, anti-convulsants and tranquilizers. Although rare, these drugs may cause a urine-based home pregnancy test to return a false positive result. In such cases, it's a good idea to take a second pregnancy test, or better yet, see your doctor for a blood test -- which is far more accurate -- and a consultation.

Suffering A Loss

A positive result on a home pregnancy test usually means you are pregnant -- or were pregnant, even if for a short time. If the pregnancy results in an early loss, or is an ectopic pregnancy -- when the egg implants in the fallopian tube or elsewhere rather than the uterine lining -- the pregnancy is not viable. It is important to see a doctor right away if you suffer from bleeding and severe pelvic or abdominal pain shortly after getting a positive result on a pregnancy test.

Going Through Menopause

Menopause, with its hot flashes, hormonal changes and irregular periods, can feel much like early pregnancy -- especially if you begin menopause early, when you could still conceivable conceive. These fluctuations in hormones can sometimes produce false positives in urine-based home pregnancy tests. To be certain of the result, visit a doctor for a blood test.

Taking An Expired Test

It's best not to dig out that old pregnancy test from a few years ago sitting in the back of your cabinet. Home pregnancy tests have expiration dates, and old test increase your chances of getting a false reading. In addition, some tests are more sensitive than others. The old adage holds true: You get what you pay for.

Fighting Cancer

In rare cases, certain types of ovarian and breast cancers can cause false positive results on home pregnancy test. In this, as in other situations, it's best to see a doctor right away to either confirm or deny a viable pregnancy.