How Accurate Are Home Pregnancy Tests?

If you’ve just taken a home pregnancy test and gotten a result you’re not sure about, this blog post is for you. We’ll discuss how these tests work and what the chances are of both false positives, where the test says you’re pregnant and you’re not, and false negatives—where the test says you’re not pregnant and you actually are.

How do home pregnancy tests work?

To understand the technology behind home pregnancy tests, you first have to know a little bit about the hormones of pregnancy. One of the earliest hormones to show up is human chorionic gonadotropin or hCG, the so-called pregnancy hormone. hCG is produced by the placenta—that supporting organ that grows as your baby grows to support its development. The placenta starts developing really early, just after the very small embryo implants into the uterine lining, so hCG also shows up early and can be detected in urine.

Home pregnancy tests are designed to detect hCG. Most of them have an absorbent filter that you pee on or dip into collected urine. The liquid is drawn across the filter and interacts with two different test areas. In the first test area the urine mixes with proteins called antibodies that stick to hCG if it’s present. In the next test area, the antibodies and hCG stick to another antibody that changes color. If there’s no hCG in the urine sample then the color change only happens in one spot that interacts with hCG antibodies that aren’t stuck to any hCG. That’s why tests with two lines have only one line for negative tests, but two lines for a positive test.

Although there are some differences in what’s commercially available, all home pregnancy tests work in a similar way. For instance, some tests detect lower levels of hCG than others are billed as accurate very early on. Digital tests use antibodies and a sensor that activates a digital readout that says “pregnant” or “not pregnant.”

When will a home pregnancy test work?

 Because home pregnancy tests detect hCG, they work best when there is a good amount of hCG present in your urine. That’s usually around the time of a missed period, so around day 28 or 29 of your cycle, if your cycle is 28 days long. Depending on when you ovulated, though, it’s possible that an at-home test will work much sooner. For example, if you know that you ovulated around day 10 or 11, it’s possible that you could have a positive test as early as day 25 of your cycle. As discussed above, some tests are more sensitive to levels of hCG, which means that they could also show a positive earlier than a less sensitive test.

What about false positives?

 False positives—when you get a positive test but are not actually pregnant—are very rare because a positive result is almost always indicative of the presence of hCG. In other words, if there is hCG present in your urine at a level that the test you’re using can detect, then it will show a positive result, and hCG indicates pregnancy. It IS possible to get a positive test that indicates a non-viable pregnancy, which can be really heartbreaking.

There are also some extremely rare circumstances in which you might get a false positive around the time of ovulation when a lot of hormones are in flux, if there is an issue with the test you’re using being expired or faulty, or if there is something more serious going on with your body. If you get a positive test, but you haven’t had intercourse, try taking another test of a different type. If that one is also positive, call your care provider.

Is it possible to get a false negative?

 False negatives—where you are pregnant but the test says you’re not—are much more common in at home pregnancy tests than false positives. The reason for this is that many people test too early. Sometimes they’ve mistaken their ovulation and conception date and it was actually later than they thought, meaning that the placental cells haven’t made enough hCG to be detected yet. Or sometimes it just takes those cells a bit longer to get it together to start producing hormones. Whatever the reason, the way to cope with a false negative is to wait a day or two and test again. Most at home pregnancy tests are most effective after a missed period, so if you wait a day or two and still don’t get your period, it’s a good idea to test again. Because of how the tests work, the lines will often get darker as hCG increases in your system, until it peaks later in your first trimester of pregnancy.

Abby Olena
Dr. Abby Olena has a PhD in Biological Sciences from Vanderbilt University. She lives with her husband and children in North Carolina, where she writes about science and parenting, produces a conversational podcast, and teaches prenatal yoga.

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