The Science Behind How Pregnancy Tests Work

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Regardless of whether you’ve bought or used a home pregnancy test in the past, it’s likely that you’ll need one at some point in your adult life. And that means it’s probably a good idea to understand the science behind how it works, right? Well, we realized that we never inquired further beyond the fact that the store-bought tests are urine-based tests and the ones run in doctor offices are blood tests. So we did some reading and decided to share the results (hah) with you. Here’s how at-home pregnancy tests work.

Home tests are designed to detect a hormone called human chorionic gonadotropin (hCG) in your urine. Your body only makes this hormone right after a fertilized egg attaches to the wall of your uterus. Usually, that takes approximately six days after possible conception occurs. And once that attachment happens, your hCG levels continue to increase, doubling every few days and reaching its peak after eight to 10 weeks.

Most urine tests for at-home use suggest waiting until at least a week after your missed menstrual cycle to allow for that initial incubation period to occur. Otherwise, you risk receiving an inaccurate reading. When you follow the directions appropriately, these tests are 97 percent accurate. Regardless of the test result received, most women visit their doctor’s office afterward to confirm the results with their more sensitive testing methods and, if pregnant, start planning the next steps of their care.

Unsplash/Concha Rodrigo

Because these tests are clearly identifying a specific hormone in your urine, false positive readings are very rare. But in the instance that you have traces of blood or a high protein count in your urine, the test could give you not-so-accurate results, in which case you should make an appointment with your doctor. Certain prescription medications can lead to false positive results as well.

False negative results, on the other hand, are much more likely due to user error. (Seriously, follow all of the directions on that packaging carefully!) Also, home pregnancy tests have expiration dates, which definitely impact results. So if your test is past its prime, trash it and buy a new one. Your hydration levels matter as well — drinking too much water can dilute the hCG hormone concentration in your urine, rendering the test results inaccurate. It’s recommended that you use the test first thing in the morning to avoid this error. Lastly, medications like antihistamines and diuretics can hinder the test’s ability to provide you with an accurate reading.

So there you have it. These tests are surprisingly straightforward, but we always feel a bit more empowered when we understand what’s going on in our bodies — and in the tests that we use to obtain that information. So whenever you need to head to that aisle in the drugstore in the future, you can now know exactly what to expect.

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