Can twins cause a false negative pregnancy test?

If you suspect you could be pregnant, one of the first things you’ll likely do is take a home pregnancy test and look closely for two lines that indicate exciting news: A baby on the way!

A negative result can be disappointing if you’ve been trying to conceive, and it’s worth testing again a few days later. Testing too early is a common reason for a false negative on a pregnancy test.

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There are some less-common reasons for a false negative, too. Believe it or not, it could happen if you’re expecting more than one baby.

How twins could cause a false negative pregnancy test

It may sound unbelievable, but it’s true: If you’re pregnant with twins or multiples, a home urine test could possibly show a negative result.

To understand why this can happen, it may be helpful to have a quick refresher on how home pregnancy tests work. They measure a hormone produced during pregnancy called human Chorionic Gonadotropin, or hCG, which can be detected in blood or urine 10 to 12 days after conception.

Home pregnancy tests instruct you to pee into a cup or directly on a test strip, then wait a few minutes for results to appear. If the test detects elevated levels of hCG in your urine, it should display a positive result, usually two side-by-side lines or a “+” sign.

The hCG hormone generally doubles every two or three days during the early weeks of pregnancy. If you’re pregnant, your urine should have enough hCG to be detected on a home test about 10 days from conception, or the day after your missed period. Some pregnant women test too early, though, especially if they ovulated late, and waiting a few days before retesting can allow hCG levels to build and show a positive result.

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Women who are pregnant with twins or multiples have even higher hCG levels in their urine than women expecting one baby. So you’d think a pregnancy test would flash “+” even more easily. But a very high hCG level can actually lead to an inaccurate result.

When you take a home pregnancy test, it’s looking for hCG levels of at least 20 milli-international units per milliliter (mUI/mL), which most women have in their urine around weeks 4 or 5 in a singleton pregnancy. (A non-pregnant woman’s hCG levels are less than 5 mIU/mL, as a comparison.)

But extremely high hCG levels (above 500,000 mIU/mL) may prevent home pregnancy tests from being able to properly interpret the result, and the read ends up negative. This is known as the “hook effect,” and it can happen if you’re further along in a singleton pregnancy (when hCG levels are significantly higher) in addition to in early pregnancy if you’re carrying twins or triplets.

Though this can definitely happen, it isn’t super common. “It’s rare, and I’ve never seen it,” says Layan Alrahmani, M.D., a board-certified ob-gyn and maternal-fetal medicine specialist and a member of the BabyCenter Medical Advisory Board.

In 2022, a case study published in the journal CureusOpens a new window described a woman with early pregnancy symptoms who received a negative result on a urine pregnancy test after going to the ER for severe nausea and vomiting. Subsequent ultrasounds revealed multiple gestational sacs, and she ended up giving birth to triplets. The authors theorized that the hook effect was the likely reason for her false negative.

Other reasons for a false negative pregnancy test

Home pregnancy tests are highly reliable – many claim to be 99 percent accurate if taken correctly. But there’s still a chance you could get a false negative.

First, always make sure you’re following the manufacturer’s instructions when taking a home pregnancy test, including waiting the full amount of time before peeking for that “+” or “-” sign. Expired tests can give a false negative, so check the expiration date if it’s an older test that’s been in your medicine cabinet for awhile, says Dr. Alrahmani.

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Most commonly, false negatives happen simply because it’s too early in pregnancy and your hCG levels haven’t yet reached that 20 mUI/mL threshold. If you think you could be pregnant but only just missed your period (or haven’t yet missed it), try testing again in a few days.

Diluted urine could also lead to a false negative early in pregnancy. For the most accurate results, take a test first thing in the morning: When you wake up, hCG levels in your urine are more concentrated.

Less often, a false negative could be caused by a molar pregnancy, a rare pregnancy complication and type of gestational trophoblastic disease (GTD) that causes a noncancerous tumor to grow in the uterus.

As with multiple pregnancies, a molar pregnancy can generate elevated hCG levels that cause the hook effect on home pregnancy tests. Molar pregnancies can have serious complications and require treatment, so reach out to your provider right away if you’re experiencing symptoms like bleeding, severe nausea or vomiting, or pelvic pain or pressure.

What to do if you think you’re pregnant but test negative

If you missed your period and are having early pregnancy symptoms, your first step is to test again a few days later. But if you keep getting a negative result, reach out to your doctor. They may recommend a blood test, which can be more useful in determining a multiples pregnancy because it shows the exact level of hCG in your blood.

Although the hook effect due to a multiples pregnancy is relatively rare – and your likelihood of having twins is just 3.2 percent in the U.S. – it’s still important to rule it out if you’re experiencing other symptoms.

As the authors of the Cureus case study note, the hook effect can be risky because it might (understandably) lead a woman to think she’s not pregnant, which could delay her prenatal care. So never hesitate to let your provider know if something doesn’t feel right. That holds true both when you’re trying to conceive and anytime after you get that positive result, too.

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