Show Reviewed by Dan Brennan, MD on May 24, 2021 It’s easy to get overwhelmed with emotions when you’re taking a pregnancy test. Your life may change forever depending on the results. Confirming those results with your doctor, even if they’re negative, is important. Pregnancy tests check the levels of human chorionic gonadotropin (hCG) in your body. This is one of the first things to develop when you’re pregnant. It's a substance that builds up when the fertilized egg embeds in the womb around 10 days after conception. At-home pregnancy test. The most common pregnancy test is an over-the-counter, at-home test. These tests check for hCG levels in your urine. While the tests claim to be 99% accurate if you follow the instructions, it’s important to follow up with your doctor. At-home pregnancy tests have some advantages compared to going straight to your doctor when you first think you may be pregnant: Your doctor can also perform a urine pregnancy test to confirm previous results. They will also talk to you about next steps to ensure your health and that of your baby. Blood test. This type of pregnancy test is done by your doctor. They’ll take a blood sample and send it off. A lab will check the levels of hCG in your blood. A blood test isn’t used frequently for pregnancy because they’re expensive and provide similar results as a urine test. A blood test is typically reserved for any concerns about the pregnancy, such as if you’re using fertility treatments. When you take an at-home pregnancy test, you may receive a false result because of a variety of circumstances. False-positive result. This is when the pregnancy test says you’re pregnant when you aren’t. This can be caused by: False-negative result. This is when the test says you aren’t pregnant but you are. Possible causes include: Pregnancy tests are a type of immunoassay, a test that measures the concentration of certain particles, molecules, in the tested material. Scientists call it the "hook effect" when a weakness of the immunoassay comes to light. The hook effect occurs when there is an abundance of analyte (what’s being measured). The analyte of a pregnancy test is hCG. During the pregnancy test, two molecules — one called an antibody and one called an antigen — become the “sandwich bread” on either side of the "filler," the hCG molecule. The number of hCG molecules that are sandwiched determine the results of the pregnancy test. Too much hCG messes up the jobs of the antibodies and antigens. The hCG molecules overwhelm the antibodies, preventing the molecular sandwich that is supposed to occur. As the pregnancy test draws to a close, all the extra molecules are washed away, leaving just a few sandwiched molecules behind. Since the abundance of hCG prevented most of the molecular sandwiches from forming, the pregnancy test falsely shows that there are low levels of hCG. Concern over results. If your at-home test came up positive, or you’ve taken multiple tests and gotten conflicting results, it’s time to see your doctor. They will perform a blood test or an ultrasound to confirm your results. Missed period. After a missed period, wait a week to use a pregnancy test. Repeat the test after a few days. The sooner you use a test after conception, the more likely you’ll get an inaccurate result. If you haven’t gotten your period but your result was negative, don’t be alarmed. There are a few causes of a missed period: Missed periods can also be the result of problems with your thyroid or ovaries. Talk with your doctor to get to the bottom of your missed periods. Mayo Clinic: “Getting pregnant.” © 2020 WebMD, LLC. All rights reserved. View privacy policy and trust info got pregnant with my third child three months after the devastating loss of my second child. To say I was nervous was an understatement. At that time, I didn’t know about the hook effect.
By Aarthi Gobinath, PhD | Published Mar 9, 2018
| Last updated Dec 10, 2019
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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 pregnancy test line or no line at all. Should you worry? Not yet. If you take a pregnancy test after about week five of pregnancy, you might experience a type of false negative called 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, when you take a pregnancy test this triggers release of a dye—that’s the dark line on a pregnancy test that means you’re pregnant! The first line on a pregnancy test is a control test that simply makes sure the antibodies and dye are working.  Why can I get a positive pregnancy test then negative?There are two reasons why you would get a positive pregnancy test and then a negative:
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). 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. How can you tell if you’re experiencing the hook effect?The hook effect can cause someone who has a healthy, normal pregnancy to get a negative pregnancy test. If you get a negative pregnancy test after a positive, how can you tell if it’s the hook effect? There’s a simple test: all you have to do is dilute your urine, which prevents hCG-βCF excess. Here’s how:
Read more about pregnancy tests: Should you take a pregnancy test? Period late, but negative pregnancy test? What’s the best pregnancy test? By Aarthi Gobinath, PhD | Mar 9, 2018 |
What is hook effect in pregnancy
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