I'm no stranger to premenstrual symptoms, but a few months ago, I experienced a couple that were new to me: My breasts were tender and my lower back throbbed with pain. I put it out of my mind until my cycle-tracking app alerted me that my period was a few days late—at which point I threw my three kids in our minivan and raced to the store for a pregnancy test. Back home, as my toddler tapped incessantly on the bathroom door, I watched as a faint second line appeared. This can’t be happening, I thought. My husband had undergone a vasectomy three years earlier.
Over the next few days, the shock settled and turned into excitement. Then, nearly a week later, I started cramping and bleeding. My doctor asked me to head to the hospital. Hours after waiting in the ER, a mandatory mask covering my fear and worry, the physician finally entered my room. “I’m sorry to tell you that you aren’t pregnant,” she said. My blood test, she explained, showed not even a hint of the hormone hCG—meaning I wasn’t just not pregnant, but I had never been pregnant at all. It seemed the test I’d taken at home had produced a false positive, and my symptoms—the PMS, the bad cramping, the lateness—despite being unusual for me, were simply part of my monthly period.
At-home pregnancy tests work by indicating whether hCG (human chorionic gonadotropin) is present in your urine, says Mary Coll-Black, an OB/GYN at St. Joseph’s Healthcare in Hamilton, Ont. The presence of hCG confirms a pregnancy, and a lack of it suggests the individual is not pregnant. If, like me, you have a positive test but you are not, in fact, pregnant, then you either have hCG in your body for another reason or something went wrong with the test.
It’s quite rare for hCG to be detectable in your urine or blood if you aren’t pregnant, but there’s a laundry list of reasons why it could happen: if you’ve recently given birth, had an abortion or experienced a pregnancy loss, or had fertility treatments where hCG was injected. It typically takes weeks for hCG to fully leave your system, says Yolanda Kirkham, an OB/GYN at Unity Health and Women’s College Hospital in Toronto. There are also some medications that could potentially create a false positive, she says, like aspirin, the anti-seizure medication carbamazepine, and methadone. There’s even some medical literature that suggests women could end up with a false positive if they test immediately after having sex, says Kirkham, because there may be a trace amount of hCG in semen.
A false positive can occur if you haven’t properly followed the instructions on the pregnancy test. For example, you’re meant to check the test results within a few minutes, while the urine is still wet, because as it dries, an evaporation line may appear as a faint second line. (You may see this referenced in online mom chats as an “evap line.”) An expired test also has the potential to skew the results; a cheap test, on the other hand, does not. Kirkham confirms that pricey tests aren’t more accurate than inexpensive ones. Even a cheap, dollar-store pregnancy test will not increase your chances of a false positive.
To get to the bottom of a false-positive pregnancy test, your doctor may ask questions and review your history. In my case, there was no reason why hCG should have been be present, a follow-up blood test showed no hCG, and I’m confident I used my pregnancy test correctly. My husband underwent a semen analysis to ensure his vasectomy had been effective (it was). The most likely scenario, said both the ER doc and my family doc, is that I had a true, unexplainable false positive—which, by the way, almost never happens. “A positive pregnancy test that turns out to be false is extremely rare,” says Coll-Black. “It’s not something we come across often.”
I’ll never know what that second line on my test was all about. What I do know is that it really mixed me up emotionally. Fortunately, most women will never go through what I did, since at-home pregnancy tests, says Kirkham, are 99 percent accurate.
If your pregnancy is in its early stages and your body has not yet produced enough hCG for the pregnancy test to detect, you could get a false negative, which means you’re actually pregnant even though the test says you’re not. The earlier you test, the higher the chances that this scenario will occur. If you get a false negative but still suspect you’re pregnant, repeat the test in a few days. If it’s negative again but your period still hasn’t arrived, book an appointment with your healthcare provider.