How accurate are pregnancy tests?

Waiting for a pregnancy test result can make even a few minutes feel unusually long. If the result is unexpected, it is natural to wonder how much you can trust it.

Home pregnancy tests are accurate when they are used correctly, according to the NHS. A positive result is almost certainly correct, while a negative result is less reliable. Timing matters. Testing too early is one of the main reasons a pregnancy test may give a negative result even though a pregnancy has begun.

Understanding what the test actually measures, and what can affect that measurement, can make the result easier to interpret.

How do pregnancy tests work?

Pregnancy tests detect human chorionic gonadotrophin, usually shortened to hCG.

The NHS explains that hCG starts to be produced around six days after fertilisation. As pregnancy develops, the amount of hCG rises, eventually reaching a level that a urine pregnancy test can detect.

Home tests vary in design. Some show lines, while digital tests interpret the result and display it on a screen. The underlying principle is similar: the test is looking for hCG in urine.

Because individual tests can differ, always follow the instructions supplied with the product you are using. This includes how to collect or apply urine, how long to wait and when the result should be read.

When are pregnancy tests most reliable?

The NHS says most pregnancy tests can be used from the first day of a missed period.

If you do not know when your next period is due, NHS guidance recommends testing at least 21 days after the last time you had unprotected sex.

Some sensitive home tests are marketed for use before a missed period. Earlier testing comes with an important limitation, though. There may not yet be enough hCG in the urine for the test to detect.

This means the same person could receive a negative result early on and a positive result several days later as hCG levels rise.

The NHS says urine for a home pregnancy test can generally be collected at any time of day. Always check the instructions for your particular test, especially if you are using a product designed for early testing.

How accurate is a positive pregnancy test?

A positive home pregnancy test is almost certainly correct if the instructions have been followed properly, according to the NHS.

That does not mean false positive results are impossible. Certain circumstances can affect hCG testing.

For example, Gloucestershire Hospitals NHS Foundation Trust notes that hCG may remain detectable for several weeks following a pregnancy. It also states that fertility treatments containing hCG can cause a positive pregnancy test.

If you receive a positive result that you were not expecting, or your circumstances make the result difficult to interpret, a healthcare professional can advise you about what happens next.

If you plan to continue the pregnancy, you can contact the NHS to start pregnancy care with a midwife.

How accurate is a negative pregnancy test?

Negative pregnancy tests need a little more caution.

The NHS specifically describes negative results as less reliable than positive ones. Two common reasons are taking the test too early and not following the instructions correctly.

A pregnancy test needs enough hCG to be present in the urine to produce a positive result. If pregnancy is at a very early stage, the concentration may still be below the test’s detection threshold.

Gloucestershire Hospitals NHS Foundation Trust also notes that a very dilute urine sample can contain a lower concentration of hCG and potentially produce a false negative result.

If your test is negative but you still think you could be pregnant, the NHS advises waiting a few days and testing again.

Why can a pregnancy test be wrong?

There is no single explanation for every unexpected result. Several factors can affect how useful a pregnancy test result is.

Testing too early

This is an important cause of false negative results. hCG may not yet have reached a detectable concentration.

Waiting until the first day of a missed period gives most home tests a better opportunity to detect pregnancy, in line with NHS guidance.

Not following the instructions

Pregnancy tests are not all identical. Using a test differently from the manufacturer’s directions or reading it at the wrong time can affect how you interpret the result.

Check the instructions before testing, even if you have used a different pregnancy test before.

Dilute urine

Gloucestershire Hospitals NHS Foundation Trust explains that dilute urine may contain hCG at a concentration too low for a test to detect, potentially resulting in a false negative.

If pregnancy is still suspected after an early negative result, its laboratory guidance recommends retesting using a first-morning urine sample.

Certain medicines or recent pregnancy

The NHS notes that some medicines can affect pregnancy test results. NHS laboratory guidance also states that fertility treatment involving hCG can cause a positive result and that hCG can remain detectable for several weeks after a pregnancy has ended.

If either situation applies to you, ask the clinician responsible for your care how to interpret your result.

Are expensive pregnancy tests more accurate?

Price alone does not tell you how useful a pregnancy test will be in your particular situation.

Different tests can vary in sensitivity, design and how the result is displayed. A digital screen may be easier for some people to read than lines, but the test still depends on detecting hCG.

Rather than judging a test by price or packaging, check its instructions and information about when it is intended to be used.

Using a test at an appropriate time and following its directions carefully are central to getting a reliable result.

Can contraception affect a pregnancy test?

Hormonal contraception does not normally prevent a pregnancy test from working.

The NHS specifically states that hormones in contraceptive methods such as the contraceptive pill do not stop a pregnancy test from detecting hCG.

No contraceptive method is completely effective. If pregnancy is possible, taking a pregnancy test at an appropriate time can help clarify the situation.

What should you do after a negative result?

If the result is negative and your period arrives as expected, you may not need to do anything further.

If the test is negative but you still think you could be pregnant, NHS guidance recommends waiting a few days and testing again.

If the second test is negative and your period has still not arrived, speak to your GP. Late or missed periods can happen for many reasons, and your GP can help if further assessment is needed.

Symptoms also matter. A negative home pregnancy test should not be used to dismiss symptoms that need medical assessment.

What should you do after a positive result?

If you have a positive pregnancy test and plan to continue the pregnancy, you can refer yourself for NHS pregnancy care with a midwife.

Finding out you are pregnant can bring excitement, uncertainty, worry, relief or several feelings at once. If you are unsure whether you want to continue the pregnancy, you can speak confidentially with a healthcare professional or sexual health service about your options.

You do not need to make every decision immediately simply because the test is positive.

When to get professional advice

Speak to your GP if you have a negative pregnancy test, repeat the test after a few days and the second result is also negative but your period has not arrived.

Seek medical advice rather than relying on a home pregnancy test if you have concerning symptoms and pregnancy is possible.

The NHS advises contacting your GP or NHS 111 if you have a combination of symptoms associated with ectopic pregnancy, such as pain low down on one side of your abdomen, vaginal bleeding or brown watery discharge, shoulder-tip pain, or discomfort when going to the toilet, and you think you might be pregnant. This advice applies even if you have not had a positive pregnancy test.

Call 999 or go to A&E if you have sharp, sudden and intense abdominal pain together with symptoms such as severe dizziness or fainting, feeling sick or looking very pale. These symptoms need urgent medical assessment.

If you are uncertain about a pregnancy test result, particularly after fertility treatment or a recent pregnancy, ask your GP, fertility team, sexual health service or another healthcare professional involved in your care for advice.