You can generally count on a home pregnancy test to be accurate when you follow the instructions. But no test is 100 percent foolproof. In very rare instances, it's possible for an at-home urine test to give you a false negative.
Experts call this very rare phenomenon the hook effect.[1] Here's more about what it is and why it can happen — plus what to do if you suspect your pregnancy results are a false negative.
What is the hook effect?
The hook effect happens only occasionally when a pregnancy test yields a false negative result — meaning it says that a person is not pregnant when they actually are.[2] (They would find out that they're pregnant by taking another test, usually at the doctor's office.)
Home pregnancy tests work by measuring a person's urine levels of the pregnancy hormone hCG (which stands for human chorionic gonadotropin). But it's possible for too-high hCG levels to actually mess up the test's reading.
"The hook effect can occur when the level of hCG is so high it interferes with the ability of the test to give an accurate result and can lead to a false low, or a false negative," explains Mark Payson, M.D., a fertility specialist and the practice director of CCRM Fertility Northern Virginia in Tysons Corner, Virginia, and a member of the What to Expect Medical Review Board.
If you're wondering how high we're talking, hCG levels typically peak at roughly 288,000 mIU/mL by around 12 weeks of pregnancy, before tapering off as things progress.
A pregnant mom's HCG levels need to be nearly twice as high, around 500,000 mIU/mL, for the hook effect to potentially happen, findings show. This can sometimes happen during molar pregnancies, which are nonviable pregnancies where the tissue that's meant to become the placenta overgrows into a mass of cysts.
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The hook effect is incredibly rare. So rare, in fact, that specific numbers are hard to come by. Experienced OB/GYNs like Dr. Payson say they've never actually known a patient to experience it, including with molar pregnancies.
"I have seen thousands of pregnancies of all types, including rare events, in 20 years of obstetrics and fertility medicine, and I have never seen the hook effect," he says.
Instead, it's more likely for a person to experience a negative pregnancy test after having a positive test. When that happens, "it's almost certainly a failed pregnancy, not the hook effect," Dr. Payson explains.
What causes the hook effect on a pregnancy test?
How exactly can a pregnancy test end up negative from high HCG levels? It can be hard to wrap your mind around, but it's easier to understand with a little bit of a deeper dive into how pregnancy tests work.
Pregnancy tests use antibodies to check for the presence of proteins from the hCG hormone. If a person's urine (or blood, if it's a pregnancy blood test conducted at a doctor's office) contains hCG, the antibodies will bind to the hCG proteins and the test will show a positive result. If the person's blood or urine does not contain hCG, there's nothing to bind to the antibodies and the test will show a negative result.
It's possible (but again, extremely unlikely) for very high levels of hCG to overwhelm a pregnancy's tests antibodies and give a false reading when a person is actually pregnant.
"If the levels of hCG are very high they essentially soak up all the test antibodies and get washed away without ever binding to the capture antibody, resulting in a false test result," explains Dr. Payson.
Should you call the doctor if you see a hook effect?
You should let your doctor know if you have a negative pregnancy test following a positive one, or if you have a negative test with obvious pregnancy symptoms (including missing your period, fatigue, frequent urination and tender or swollen breasts).
Though it can be difficult to process, a positive test followed by a negative one could indicate a failed pregnancy. That said, if you're having clear pregnancy symptoms and a negative reading, your doctor can try testing a more diluted form of your urine or blood, or simply try a different type of pregnancy test, says Dr. Payson.
The hook effect can potentially cause an expecting mom to get a negative pregnancy test result, but the phenomenon is very rare. If you have questions or concerns about your pregnancy test or aren't sure how to interpret the results, talk to your doctor or midwife.
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