How Does a Home Pregnancy Test Work?

Pregnancy tests with a calendar marking days (How Does a Pregnancy Test Work?)

Verywell / Ellen Lindner

It's been a few months since you started trying to conceive and this time around, your period is already a day late. It's not unheard of for your cycle to be off by a few days, so you're not sure if this could be a pregnancy. You decide to take a home pregnancy test to find out.

As you take one of the tests out of its wrapping, you start to wonder how in the world peeing on this stick can possibly tell if you are pregnant. It's a fair question—and one with a scientific answer. In general, home pregnancy tests operate by detecting the presence of a hormone that is only produced after a fertilized egg implants in your uterus.

This functionality can limit home pregnancy tests in terms of accuracy and timing. Your body only produces this hormone at a very specific time, so it's possible for a home pregnancy test to miss the mark and give you a negative result when you do actually have a baby on the way. Understanding how these tests work can help you better use them to determine if you're pregnant.

What Is an At-Home Pregnancy Test?

An at-home pregnancy detects the presence of human chorionic gonadotropin (hCG) in your urine to determine whether or not you are carrying a pregnancy. Your body begins producing this hormone after an embryo implants in your uterus. In most cases, HCG is detectable in your urine by the time your period is due. "The exact timing of implantation can vary, but it typically occurs eight to ten days after ovulation," says Sam Rahman, MD, a board-certified OB/GYN and clinical assistant professor of OB/GYN at Northwestern Feinberg School of Medicine in Chicago.

When you unwrap a pregnancy test, you'll often find a stick with a handle, test window, cap, and an absorbent end. The test end will have a removable cap and is often marked with downward pointing arrows. You'll see a line near the tip of the test end; it's important to avoid getting any liquid above that marking.

In order to take a pregnancy test, your urine will need to come into contact with the test end of the stick. You can do this by peeing into a clean container to collect a urine sample (some tests will include a cup for you to use) or by peeing directly onto the test itself. If you decide to collect a urine sample, you'll have to dip the pregnancy test into the cup. No matter which method you choose, it's important to carefully read the instructions before taking a sample, and if your test comes with a cap, be sure to replace it before waiting for results.

Once you're ready, lay the stick horizontally on a flat surface and wait the amount of time listed in the directions (usually about three minutes). Then check the test. Two lines means that your result is positive.

If you only see the control line, the test is negative. If the control line does not show up at all, the test didn't function correctly. Discard it and take another test. Some newer tests will say "pregnant" or "not pregnant" in the test window for an easier detection than the lines. If you have doubts about your results, be sure to reach out to your OB/GYN or healthcare provider for a blood test to confirm.

How Do At-Home Pregnancy Tests Work?

HCG is sometimes called the pregnancy hormone, because your body only produces it when you are pregnant. When one of your eggs (or two in the case of twins) is successfully fertilized and attaches itself to your uterine wall, HCG works to begin building your placenta and preparing your womb to house a tiny new occupant. "HCG can be detected in the blood eight days after implantation, but it takes longer to show up in urine," says Kim Langdon, MD, an Ohio-based OB/GYN at Medzino with over 20 years experience.

HCG levels rise steadily after implantation. Around the time of a missed period, this hormone is typically detectable in urine. Although some tests claim to give you an accurate result sooner, there are no standard criteria to support these claims. "Because HCG comes in many forms and sub-units, some tests are more sensitive than others," notes Dr. Langdon. "Most report 99% accuracy but this has not been confirmed by rigorous testing."

If you take a test too soon, your HCG levels might not be high enough to reflect a positive result. If your period still has not come, you can test again the next day. Remember that you only produce HCG during pregnancy, so even a very faint second line is a positive result. If you continue to test, those lines should get darker each day.

Limitations of At-Home Pregnancy Tests

Despite their general reliability, there are some limitations to consider when using home pregnancy tests.

First, there is always risk of getting a false negative. A test will show negative if there is not enough HCG in the urine. This can happen if you test too early, if you don't use enough urine, if you dip the stick for too short a time, or if your urine is diluted from having consumed lots of fluids. A negative simply means that the test didn't detect any HCG, but it doesn't necessarily mean that you're not pregnant. "If there is a negative and pregnancy is still suspected, you should either see your healthcare provider or recheck in a week," advises Dr. Rahman.

Although false positives are rare, taking certain fertility medications that have HCG in them can turn home pregnancy tests into a potentially unreliable method. Waiting too long to read your results can also leave streaks on the tests, called evaporation lines. If the evaporation line is in the same spot where the positive line should be, it can look similarly to a faint positive.

Early testing may reveal a pregnancy that ends before you would have even known that you were pregnant. Some people would prefer not to know about these early losses, called chemical pregnancies. But, if you have been struggling to conceive, knowing about these miscarriages might help you identify what your issue is, so talk with your healthcare provider.

A Word From Verywell

In general, at-home pregnancy tests are a reliable way to check whether you're pregnant before meeting with your OB/GYN, midwife, or healthcare provider. They work by detecting the hormone HCG, which is only in your body during pregnancy. Once you have a positive pregnancy test, be sure to reach out to a healthcare professional. Not only will your provider confirm the pregnancy, but they will find out more about your baby's health at the first ultrasound.

5 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Elisa Cinelli
Elisa is a well-known parenting writer who is passionate about providing research-based content to help parents make the best decisions for their families. She has written for well-known sites including POPSUGAR and Scary Mommy, among others.