From the moment you see those two little lines on a pregnancy test, you may experience a rush of emotions. And waiting for a doctor's visit to confirm the news can feel like torture.

Many women choose to take a second (or third!) at-home test. Occasionally, though, those tests may have different results.

What does it mean when one pregnancy test is positive and the other is negative?

While many at-home pregnancy tests promise a 99% accuracy rate, they're not perfect. Occasionally, 'false positives' occur—when you get a positive result, even though you're not actually pregnant.

We're breaking down everything you need to know about false positive pregnancy tests.

Are they common?

Thankfully, no.

"False positives are fairly unusual," says Mary Jane Minkin, M.D. and Clinical Professor of Obstetrics at Yale School of Medicine. "Among the more common explanations are women who wait longer than the recommended time to read tests, and with evaporation, another line can appear (which is not a true positive, but just a function of the test sitting there for more than the recommended time.)"

Can medications cause a false positive?

It's possible. "Fertility drugs or other medications that contain HCG might interfere with home pregnancy test results," says the Mayo Clinic. "However, most medications, including antibiotics and birth control pills, don't affect the accuracy of home pregnancy tests."

What else could cause them?

A serious urinary tract infection (UTI) can cause a false positive, according to Gloucestershire Hospitals.

"Occasionally, especially given the sensitivity of these tests, the test could stay positive for a few days, and then turn negative," adds Dr. Minkin. "What that can signify is something that folks call a chemical pregnancy. Indeed, an egg was successfully fertilized, started to implant and then stopped developing, and what will happen then is the early pregnancy will pass out from the uterus. It may just seem like a heavy period a few days late. It is really a very early miscarriage. So, the pregnancy test was not a false positive."

"Knowing that you could get pregnant is really a very positive thing—the sperm made it through to meet up with the egg, and fertilization took place. In general, what causes the very early miscarriage is that particular sperm meeting up with that particular egg. They didn't end up perfect genetically, and the embryo stopped developing—but that should have no bearing on you conceiving subsequently—and you know you could get pregnant," she says.

Is there anything you should do to make sure you don't get a false positive?

The easiest way to avoid them is to read the test in the suggested timeframe, says Dr. Minkin.

And remember: they're really not that common. If you're feeling anxious, you can repeat the test in a few days.

What should you do after getting a positive test reading?

First, make sure you book an appointment with your healthcare provider to confirm the results.

While you're waiting for that appointment, Dr. Minkin recommends starting a prenatal vitamin with extra folic acid. "Pregnancies that are conceived while the woman is on extra folic acid have a lower chance of having neural tube defects and other congenital anomalies," she says.

If you're not actually pregnant, a prenatal vitamin won't hurt you. And if you pregnant, your baby will benefit from the early folic acid.