Here are all the reasons your pregnancy test may not actually be right.

By Bianca Mendez
Zave Smith/Getty Images

For roughly 10 percent of women in the U.S., getting pregnant is hard work—so you can only imagine the sadness and despair if they learn that their positive pregnancy test was actually a false alarm.

While rare, false positive pregnancy test and false negative pregnancy test results are entirely possible. Read on to find out what can cause them.

How Pregnancy Tests Work

First, let's talk about the science behind pregnancy tests: At-home pregnancy tests work by detecting human chorionic gonadotropin (HCG), a hormone created by placenta cells after an egg is fertilized, says Mary Jane Minkin, M.D., clinical professor of obstetrics, gynecology and reproductive sciences at Yale University School of Medicine. That hormone is excreted in your urine, which is how most tests determine whether or not you're pregnant. For a pregnancy test to fail, it's usually not because the test itself is faulty, but because of abnormal levels of HCG hormones in your urine, says Dr. Minkin.

Possible Causes of a False Positive Pregnancy Test

As mentioned, false pregnancy test results are rare, since the tests are generally considered 99-percent accurate—but certain factors can alter their results. (If you're trying to get pregnant, also check out these Best Sex Positions to Get Pregnant.)

1. You had a miscarriage.

If you recently had a miscarriage, because you were pregnant, there may be leftover HCG in your system that could trigger a false positive result. When a pregnancy isn't developing properly and results in a miscarriage, your body won't release HCG as it normally would, says Dr. Minkin. But traces of the hormone could still be in your system afterward, which can lead to a positive pregnancy test. "It takes a while for the HCG to be cleared from the body," says Dr. Minkin. In fact, HCG can linger for up to six weeks post-miscarriage, according to the American Pregnancy Association. (Related: How I Learned to Trust My Body Again After a Miscarriage)

2. It's a side effect of medication.

Certain medications, like antidepressants and those that treat seizures, could interfere with your pregnancy test results. "Particular medications bind to the marker in the test as HCG would, making it look positive," says Dr. Minkin. Again, this is unusual, but if you're trying to get pregnant and are on medication, it's important to talk to your doctor about its potential side effects and inferences.

3. You're doing fertility treatments.

Ironically, fertility treatments can also mess with your test. "Some women receive an injection of the HCG hormone to help release the egg," explains Dr. Minkin. In this case, the HCG  triggering the positive result is likely coming from the injection, and not from an actual pregnancy. If you are on fertility treatments and get a positive result, your doctor should conduct a blood test to confirm whether or not you are indeed pregnant. (Learn more about What Ob-Gyns Wish Women Knew About Their Fertility.)

3. It's a chemical pregnancy.

In a chemical pregnancy (the clinical term for a very early miscarriage), the sperm fertilizes the egg and your body starts making HCG, but soon the embryo stops developing and never turning into a fetus, explains Dr. Minkin. The evidence of fertilization, however—that small amount of HCG—could cause a false positive pregnancy test.

"Basically, the sperm and egg got together and started making some of the HCG, but then the embryo didn't implant properly," says Dr. Minkin. However, the HCG levels can be high enough that a pregnancy test can pick them up. (Related: Should You Eat Based On Your Menstrual Cycle?)

Possible Causes of a False Negative Pregnancy Test

1. You took the test too early.

False negative pregnancy test results do happen as well; however, in this case, it's mostly due to human error, says Dr. Minkin. "Taking a test too early can result in a false negative because the embryo doesn't start making detectable levels of HCG right away," she says.

Typically, women ovulate on the fourteenth day of their menstrual cycles, but depending on the length of your cycle, you could ovulate a few days earlier or later. (Here are a few reasons your period could be irregular.) When you ovulate, your body releases an egg for fertilization to occur. It takes about eight to nine days after fertilization for the egg to develop into an embryo, implant onto the uterine wall, and be producing enough HCG to be detectable, says Dr. Minkin.

However, if you ovulated a little later in your cycle then you estimated, your HCG levels may still be low at the time of the test, says Dr. Minkin. A good rule of thumb is to take a test if you notice your period is late; if it's negative, try again two or three days later or wait to see if your period shows up, she says. If it's been a week and still no period, re-test or go to a doctor and ask for a blood test.

2. It's an ectopic pregnancy.

An ectopic pregnancy is indeed a pregnancy, but the embryo develops in areas like the Fallopian tubes or cervix instead of the uterus. "With ectopic pregnancies, the HCG level is lower than in a pregnancy that is developing normally," says Dr. Minkin, so you may actually be pregnant even though a test can turn up negative.

While they're a rare occurrence, ectopic pregnancies could be dangerous. Common symptoms of an ectopic pregnancy can be abdominal pain and a missed period but, according to Dr. Minkin, the only way to be certain is through an ultrasound. If you think you could be having an ectopic pregnancy consult your doctor.

What to Do If You Think You Had a False Pregnancy Test Result

If you're unsure whether or not you're pregnant, you can always contact your healthcare provider and ask for a blood test—especially if you get a positive test result.

"A major advantage of blood tests is that they can measure how much of the HCG is present," says Dr. Minkin. But rest assured: Pregnancy tests do live up to their 99-percent accuracy claims.

For the most accurate result, take the test right when you wake up, because that's when your urine is most concentrated, suggests Dr. Minkin. If you're unsure about the result, it's a good idea to wait a couple of days and take another one (or go see your doctor).

Dr. Minkin also suggests taking a test such as First Response because they can detect around the same amount of HCG hormones as a blood test would. (Research published in 2011 supports First Response's 99-percent accuracy claim for both the manual and digital test.) "But as long as you stick to the directions on the package, you should do OK," says Dr. Minkin.

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