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Fertility basics

False Negative Results when You’re Pregnant: Is the Hook Effect Real?

False negative results can be stressful part of your initial steps in pregnancy. You’re sure that you’re pregnant because you recently had a positive pregnancy test, but now a few weeks later, you see a faint line or no line at all. Should you worry?

Not yet.

If you take a pregnancy test after about five weeks into your pregnancy, you might experience the hook effect. To explain why this happens, this post will cover how a pregnancy test works, what the hook effect is, and what you can do.

How does a home pregnancy test work?

A home pregnancy test measures amount of hCG in your urine. This type of a test is a sandwich enzyme immunoassay. It sounds complicated, and the chemistry of it is, but if you’ve made a peanut butter and jelly sandwich before, then you’ll understand how this test works.

HCG is a really big molecule that is made of two parts: the alpha part and the beta part. Let’s call the alpha unit of hCG “the peanut butter” and the beta unit “the grape jelly”.  To detect both parts of hCG, a pregnancy test loads the stick with two different antibodies—let’s call them “the bread slices”. The first antibody gloms on to the alpha unit (“peanut butter”) and the second antibody gloms on to the beta unit (“the grape jelly”), which forms a chemical sandwich. Once hCG is successfully sandwiched, this triggers release of a dye—that’s the dark line which says you’re pregnant! The second line on a pregnancy test is a control test that simply makes sure the antibodies and dye are working.

Why can I get a false negative after a positive pregnancy test result?

So, you might expect that because the further along in pregnancy, the darker the line on your pregnancy test. But this is not the case, and it’s because of the antibodies loaded in the pregnancy test.

Just as you can have other variations of peanut butter and jelly sandwiches besides the one with classic grape jelly—say, with strawberry jam—there are also variations of hCG. Why does this matter? The relative amounts of these different variations change over the course of pregnancy, and this messes with the antibodies in your pregnancy test.

Pregnancy tests are good at measuring intact hCG—or when there’s peanut butter AND grape jelly—which is great during the first few weeks of pregnancy. However, around week five, the dominant form of hCG in your urine is hCG-βCF, which is a version of hCG that is all beta unit and no alpha, or all strawberry jam and no peanut butter. That’s a problem for a pregnancy test. The test gets overwhelmed with all the beta fragments (or strawberry jam), which interferes with test’s ability to find intact hCG (or peanut butter and grape jelly).

So, instead of a linear relationship where the line gets darker the more pregnant you are, the pattern actually looks more like a hook or curve, with a darkening line from weeks three to five of pregnancy, then a fading line thereafter. This means it’s possible to get a positive pregnancy test, and then a few weeks later get a negative one—even though you’re still pregnant! Scientists have even tested this by taking a positive urine sample, immediately adding pure hCG-βCF, and then observing a negative result.

So, I think I’m seeing a false negative in my test. How do I know for sure?

Unfortunately, even though we know that hCG-βCF interferes with accurate results, companies are reluctant to re-do their products. But, if you really want to take that second pregnancy test, diluting your urine to prevent hCG-βCF excess can get around the hook effect. Try diluting your urine with some water, dip a new pregnancy test in, and the test line should now be darker.

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