You have all the signs — a missed period, nausea and vomiting, sore boobs — but the pregnancy test comes back as negative. Even the blood test in your doctor’s office says you’re not pregnant.

But you know your body better than anyone else. You continue to have symptoms and insist that you may be pregnant. A few weeks later, your doctor gives you another ultrasound scan. It turns out you are pregnant!

This scenario is pretty rare, but it can definitely happen.

So why were the pregnancy tests negative? One explanation for a false negative pregnancy test is what’s called the hook effect. It’s not common but sometimes this effect leads to urine and blood tests giving the wrong result.

This error might happen even after you’ve had one positive pregnancy test and test again a couple days later. No, you’re not going crazy — and you’re not necessarily miscarrying when this happens, either.

Most people — including many health professionals — haven’t even heard of the hook effect. It’s a science term for a rare lab test glitch that causes a faulty result. The hook effect is also called a “high-dose hook effect” or “prozone effect.”

Technically, you can have a hook effect with any kind of medical lab test: blood, urine, and saliva. The hook effect will give you a false negative, when you should have a positive result.

It happens when the test is, well, too positive.

Let us explain.

This might sound counterintuitive, but it’s kind of like when you have too many options for jeans or breakfast cereal, so you can’t choose one to buy at all.

Another analogy for you: A tester who counts tennis balls by catching them can handle a few dozen tennis balls at a time. But suddenly throw hundreds of tennis balls at her, and she’ll duck for cover and not catch any at all. Then, if someone else decides how many tennis balls are on the court by counting how many the tester caught, they’ll incorrectly say none.

Similarly, too many of one kind of molecule or many different kinds of the same molecule in the body can mess up a lab test. The test isn’t able to properly attach to any or enough of the right kind of molecules. This gives a false-negative reading.

The hook effect incorrectly gives you a negative result on a pregnancy test. This can happen during early pregnancy or in rare cases — even into the third trimester, when it’s pretty clear you’re preggers.

During pregnancy your body makes a hormone called human chorionic gonadotrophin (hCG). You need this hormone for a healthy pregnancy. It’s first made when the fertilized egg burrows into the wall of your uterus during implantation and increases as the embryo grows.

Pregnancy tests pick up hCG in the urine or blood. This gives you a positive pregnancy test. Your blood might have some hCG as early as eight days after ovulation.

This means you could get a positive pregnancy test at the doctor’s office, or even on an at-home test in some cases, even before you’ve missed your period! Ah, science.

But hCG is also responsible for the hook effect giving you a false-negative pregnancy test. The hook effect happens when you have too much hCG in your blood or urine.

How is this possible? Well, the high levels of hCG overwhelm the pregnancy test and it doesn’t bond with them correctly or at all. Rather than two lines saying positive, you get one line that incorrectly says negative.

You wouldn’t think you could have too much hCG any more than you could be too pregnant. What does that even mean?

But if you’re pregnant with twins or triplets (or more!) you may have more hCG in your blood and urine. This is because each baby or their placenta is making this hormone to let your body know that they’re there.

The hook effect is more common when you’re carrying more than one baby. The high level of hCG hormone confuses the pregnancy tests.

Fertility drugs and other medications with hCG can also raise levels of this hormone. This might mess up your pregnancy test results.

On a very serious note, another cause of high levels of hCG is a molar pregnancy. This pregnancy complication happens in about 1 in every 1,000 pregnancies. A molar pregnancy happens when the cells of placenta grow too much. It may also cause fluid-filled cysts in the womb.

In a molar pregnancy, the fetus may not form at all or there might be a miscarriage very early in the pregnancy.

A molar pregnancy is also a serious risk to the mother. See your doctor if you have any of these signs:

  • a negative pregnancy test after a previous positive test
  • negative pregnancy tests with symptoms of pregnancy, such as missed period, nausea, or vomiting
  • severe nausea and vomiting
  • pelvic pain or pressure
  • bright red to dark brown vaginal bleeding after a positive pregnancy test

The hook effect isn’t just misleading. It can be harmful for both you and your baby. If you don’t know you’re pregnant, you may unintentionally do harm by taking certain medications, drinking alcohol, or using other substances.

Additionally, you may not be aware that you’re having a miscarriage if you don’t know that you’re pregnant. Or you may not know that you were even pregnant until you have a miscarriage. There’s no way around it — both of these scenarios can be emotionally and physically tough.

You need medical care during and after a miscarriage. A miscarriage at any time during a pregnancy can leave some remnants in the womb. This can cause infections, scarring, and even some kinds of cancers.

Remember, we’re not saying a negative test due to the hook effect necessarily means miscarriage. But if you do miscarry, a doctor can check for any leftover tissue with an ultrasound scan. You may need to have a procedure to remove the tissue.

Some doctors say you may be able to “MacGyver” a pregnancy test to avoid the hook effect.

One way to do this is to dilute your urine before using a pregnancy test. After peeing in a cup, add a few tablespoons of water to your urine so it becomes lighter in color.

This might work because it reduces how much hCG you have in your urine. You’ll still have enough of this hormone for the pregnancy test to “read,” but not so much that it’s overwhelmed.

But then again, this might not work. There’s no research proving this method.

Another way is to avoid doing a urine pregnancy test first thing in the morning. Many at-home pregnancy tests advise you to take the test after waking up because your urine is more concentrated then. This means more hCG.

Instead, try waiting until later in the day to take a pregnancy test. In the meantime, drink plenty of water as another dilution technique.

These tips may not work for everyone who gets a false-negative pregnancy test.

Getting a false-negative pregnancy test because of the hook effect is rare. False-negative test results can happen for many reasons.

One older study that tested 27 different kinds of at-home pregnancy tests found that they gave false negatives almost 48 percent of the time. That’s huge! But that also wasn’t due to the hook effect most of the time.

You might get a false-negative pregnancy test for other reasons. Some at-home pregnancy tests aren’t as sensitive to hCG as others. Or you may take a test too early. It takes time for the hCG hormone to appear in your urine.

Talk to your doctor if you think you’re pregnant even after you get a negative pregnancy test. Make a follow-up appointment a few weeks later and ask for another test and an ultrasound scan.

If you have a molar pregnancy, you need urgent treatment and careful monitoring. Don’t ignore any symptoms or changes to your body.

You know your body best. Let the doc know that the tests might be wrong if you feel that you might be pregnant. Don’t feel ashamed or let anyone tell you it’s “all in your head.” Sometimes, your intuition is spot-on. And if it’s not this time, you have nothing to lose by double-checking.