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2nd March, 20213 min read

When to take a pregnancy test

Medical reviewer:Dr Ann Nainan
Author:Kathryn Reilly
Last reviewed: 02/03/2021
Medically reviewed

All of Healthily's articles undergo medical safety checks to verify that the information is medically safe. View more details in our safety page, or read our editorial policy.

If you think you might be pregnant, you’ll want to know for sure as soon as possible.

Some pregnancy tests can be used before you miss a period, as soon as 8 days after having unprotected sex. But the results may not be accurate.

So read on to learn more about how pregnancy tests work and when it’s best to use one.

How do pregnancy tests work?

Pregnancy tests work by checking for the presence of the ‘pregnancy hormone’, called human chorionic gonadotropin (hCG), in your wee.

Your body starts to make small amounts of hCG as soon as a fertilised egg implants itself in the wall of your womb (uterus). This happens 6 to 12 days after the egg has been fertilised by sperm from a penis.

hCG tells your body to carry on making progesterone. This stops you from having a period, so that the egg can start to grow in your womb.

How early can you take a pregnancy test?

Some very sensitive types of pregnancy test can be used as soon as 8 days after having unprotected sex – but they’re not always accurate. This is because you’ll only have very low levels of hCG in your wee at this early stage. So you could get a negative (not pregnant) test result, but actually be pregnant.

In fact, it’s best to wait at least 19 days after you think you might have become pregnant, to allow time for your levels of hCG to be high enough to be detected.

Waiting until the day you expect your next period will give you the highest chance of an accurate result. If you don't know when your next period is due, do the test at least 21 days after you had unprotected sex.

If you get a negative result, but still think you could be pregnant, wait another 3 days before taking another test. Talk to your doctor if you get another negative result, but your period doesn't start.

Are early pregnancy tests reliable?

Doing a test too early does increase your chances of a ‘false negative’ pregnancy test – when it says you’re not pregnant, but you actually are – because your levels of hCG are too low to be detected.

False positives are also possible, but they’re much less common. In some cases, they’re due to very early miscarriage.

What to do next

This is the time to get in touch with your doctor or healthcare provider.

Key points

  • doing a pregnancy test too early may give you an inaccurate result
  • it’s more common to get a negative result when you’re actually pregnant than the other way around
  • it’s best to wait until the day your period is due before doing a test – or at least 21 days after you have unprotected sex
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Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.