There are few circumstances more confusing than a missed period and a negative pregnancy test. Are you pregnant or not?

Although modern at-home pregnancy tests are usually reliable, they can still give inaccurate results (despite that 99% accuracy rating on the box). So if your period is still missing, but you’re holding a negative test, you’re probably wondering if it’s a false negative.

There are several common and some not-so-common reasons false negative pregnancy tests occur, and you actually are pregnant. But first, a quick note on how those home pregnancy tests work.

Detecting hCG, the pregnancy hormone

The “pee stick” you’re using is designed to detect the pregnancy hormone human chorionic gonadotropin (hCG) after it passes a certain level of concentration in your bloodstream and urine. If enough hCG is present in your urine, a chemical reaction occurs between the hCG and the chemicals in the pregnancy test, and that famous second line appears, indicating pregnancy.

However, there are times hCG is present in your urine, and you’re pregnant, but the test doesn’t detect it. If the test fails to detect the hCG, that second line won’t appear, and you’ll get a false negative result. There are several reasons this happens.

6 reasons for false negatives

1. You’re pregnant, but you tested too early

The most common reason for a false negative pregnancy test? You didn’t wait long enough before testing.

Your body doesn’t immediately begin producing hCG after fertilization of an egg—it comes from the cells of your placenta.1 That means hCG isn’t present until after implantation (about 6-12 days after fertilization) and the formation of the placenta. After that, the amount doubles every two or three days.2

If you test before implantation or only a few days after, there won’t be enough hCG in your urine for the pregnancy test to detect it.

2. You’re pregnant, but you drank too much water

Overly diluted urine is problematic as well, so avoid drinking too much before testing. This is why pregnancy test instructions will advise you to use “first morning urine,” which is much more concentrated.

3. You’re pregnant, but you didn’t test correctly

User error is common with home pregnancy tests, and can lead to false negative or inconclusive results. Make sure to carefully read the instructions that come with the pregnancy test, as they vary between brands. They should tell you:

• How many seconds to hold the stick in your urine stream or in a cup of collected urine.
• How to place the stick while you wait for results (ex: horizontal on a flat surface).
• How many minutes it takes to get results.
• After how many minutes you shouldn’t check for results.
• Where the control line is.
• What a positive result looks like.
• What an invalid result looks like.

If you get an invalid result after testing (which usually means the control line doesn’t appear), throw away the test and repeat.

4. You’re pregnant, but the test was expired

You can still get an accurate result using expired pregnancy tests, but it’s best to avoid using them anyway. Over time, the chemical in the test that reacts to the presence of hCG loses sensitivity. Eventually, the pregnancy test will no longer be able to detect the hormone consistently or at all, even if it is present.3

5. You’re pregnant, but it’s an ectopic pregnancy

An ectopic pregnancy occurs when a fertilized egg implants outside the uterus, most often in the fallopian tube. When that happens, it’s known as a “tubal pregnancy.”

Tubal pregnancies are a serious medical issue, since the growing embryo will eventually burst the fallopian tube, causing internal bleeding. It’s rare, but it happens. And in less than 3% of ectopic pregnancies, a pregnancy test will come back negative.4,6

Here are some warning signs to look for if you suspect you are pregnant, but received a negative test result: 5

• Pain in your lower abdomen or on one side
• Dizziness, lightheadedness, or fainting
• Vaginal bleeding or spotting
• Nausea and vomiting
• Shoulder pain

If you suspect you are experiencing an ectopic pregnancy, make an appointment with your doctor as soon as possible. An ultrasound can determine the location of a developing embryo. If it has implanted outside the uterus, immediate medical treatment is needed.6

6. You’re pregnant, but you have too much hCG (the hook effect)

Another rare cause of a false negative pregnancy test is the “hook effect.” It may seem counterintuitive, but the hook effect occurs if you have too much hCG in your bloodstream or urine.

Simply put, if the amount of hCG overwhelms or confuses the chemicals in the pregnancy test, they won’t bind with any of the hCG present.7

This is more common if you’re carrying multiples and more than one placenta is producing hCG, or if you’re taking fertility drugs. It’s also possible if you’re experiencing a molar pregnancy, a rare complication that occurs in about 1/1000 pregnancies.

In a molar pregnancy, there is abnormal growth in the cells that would normally form the placenta. This leads to increased hCG production, cysts, early miscarriage (if a fetus is forming at all), and possible cancer if left untreated.8 Mayo Clinic lists these symptoms as indicative of a possible molar pregnancy.

Overcoming the hook effect

So how can you work around a possible hook effect? There’s no research to back up this method, but some women have claimed that drinking water and diluting their urine (and therefore lowering the concentration of hCG in a urine sample) before testing again can produce the positive test they suspected.7

Conclusion

Whether you tested too early, drank too much water, made a user error, encountered the hook effect or even discovered an ectopic pregnancy, it’s important to identify a false negative pregnancy test as soon as possible.

If you’re wondering whether you tested too early, use our pregnancy test calculator to determine the best date to get an accurate result. Or make an appointment with your provider, who can administer a more sensitive medical-grade pregnancy test. This could be a blood test, which is more accurate than a urine test.

Above all, trust your gut and your knowledge of your body, and keep investigating. Your health and your peace of mind matter, especially after an unexplained missed period. Here at She Might, we want to help you achieve wholeness—body, mind, and spirit—no matter the pregnancy test result.