How Do Pregnancy Tests Work?

Taking a pregnancy test often comes to mind when you want to check you’re pregnant, or want to make sure you’re not. Many women assume that a late menstrual cycle is an easy indication of pregnancy, but there are many reasons why a cycle doesn’t run like clockwork. In short, a period that’s a week late may not actually indicate you’re pregnant at all.

This is where a pregnancy test comes in.


If you’re trying to conceive and are sexually active, it’s great to know how pregnancy tests work, the types of pregnancy tests that exist and when you can use them.

Types Of Pregnancy Tests

1. Testing At Home

The first and most common form of pregnancy tests you likely know about are at-home tests. These can be bought in any grocery store – even sometimes at the gas station down the street. They’re cheap and widely available for any woman to use. They’re also more private, which some women need and prefer.


However, it’s important to note that home pregnancy tests aren’t always accurate. These tests work by monitoring your levels of hCG, or Human Chorionic Gonadotropin. When this hormone is present in your urine, it means that a fertilized egg has attached to your uterus. At-home tests can sometimes miss lower levels of hCG, ruling tests inaccurate.

Other common reasons why an at-home test may be inaccurate include

  • You took the test incorrectly.
  • The test is past its expiration date.
  • Your urine is diluted due to hydration (that’s why it’s recommended to use the first morning urine when testing).
  • You’re taking medications that may invalidate pregnancy test results.

2. Testing At Doctor’s

There are two types of pregnancy tests you can take via a doctor – blood and ultrasound tests. Doctors can also perform urine tests.

Blood tests are less common, though more effective at determining pregnancy. Through blood, doctors can more accurately see how much hCG you are releasing, confirming whether or not you are pregnant.


Ultrasound tests are less frequent because of their invasive nature. However, an ultrasound can accurately pick up a fertilized egg that has grown large enough and/or a smaller fetus.

How Long Do I Wait?

After you’ve missed a period, how long to wait? Again, sometimes a woman’s cycle falls off track – it could be because of your diet or stress in your life, but we’ve all been there. However, the best rule of thumb is that it’s better to be safe rather than sorry. Plus, I imagine that most of you will be in a hurry to test, to ensure you are (or aren’t) pregnant, so will be testing as early as possible.


Your body begins to release the hCG hormone about six days after fertilization, but this process can sometimes start sooner or take even longer to fully begin. If your cycle is more than seven days late, this can indicate a possible pregnancy.


It’s always easier to take an at-home pregnancy test and discover some basic information. Here are your best recommendations based on each potential outcome:

  • If an at-home test is negative, wait for another week. If your period is still irregular, take another at-home test or visit your doctor for a urine or blood test. This can fully confirm if you aren’t pregnant.
  • If an at-home test is positive, visit an OBGYN to have a quantitative hCG test performed. This can fully confirm pregnancy, as well as identify potential issues with a pregnancy in its early stages.

Also, be aware of the rare change of a false positive – this means an at-home pregnancy test says that you’re pregnant when in reality you aren’t. This can happen due to medications you’re taking or protein or blood being present in your urine.