How accurate are pregnancy tests?

Home pregnancy tests are very good at what they do. The NHS says a positive result is almost certainly correct. But how far you can trust a negative result depends a lot on when you test and how you do it.

What does “99% accurate” mean?

Most tests say they’re over 99% accurate from the day your period is due. That figure comes from the test makers’ own lab testing of urine samples, not from people testing at home. It tells you how well the test picks up the pregnancy hormone hCG when there’s enough of it to find, used exactly as the instructions say.

It doesn’t promise that every pregnancy will show up on that day, and it doesn’t apply to testing before your period is due. The NHS doesn’t give a percentage. It says a positive result is almost certainly correct, but a negative result is less reliable.

Why positives and negatives aren’t equally reliable

A positive result means the test has found hCG. Outside pregnancy, your body doesn’t normally make enough hCG for a home test to find, so when a test finds it, you’re almost always pregnant or have been very recently.

A negative result only means the test hasn’t found enough hCG yet. That can be because you’re not pregnant, but it can also be because the pregnancy is too new, or the test isn’t sensitive enough for how early you are. That’s why a negative is less reliable, especially early on.

What the research shows

Tests look for hCG, which your body only starts making once a fertilised egg has implanted in your womb. Implantation happens around 6 to 12 days after ovulation, and hCG then takes a few days to build up.

Because everyone ovulates and implants at a slightly different time, some pregnancies don’t have enough hCG to find even on the day a period is due. One study found that on the day women expected their period, about 1 in 10 pregnancies hadn’t implanted yet. The US Food and Drug Administration (FDA) says 10 to 20 in every 100 pregnant women won’t get a positive result around the day their period is due.

The most common reason is that ovulation happened later than you thought. Ovulation usually happens 10 to 16 days before your next period, so if your cycles vary, it’s easy to test earlier in a pregnancy than you realise.

Why the numbers don’t seem to match

“Over 99% accurate” and “10 to 20 in 100 pregnancies missed” sound like they can’t both be true, but they measure different things. The 99% is about whether a test detects hCG in lab samples that contain it. The research figures are about real pregnancies, some of which simply haven’t made enough hCG yet on the day the period is due. A test can’t find what isn’t there.

Testing early

Some early tests can be used from 5 days before your period is due, but they pick up fewer pregnancies the earlier you test. Read how accurate are early detection tests? for what the box claims really mean.

Things that can affect your result

When you test. The single biggest factor. The later you test, the more hCG there is to find.
How you do the test. Each brand is a little different: how long to hold it in your pee, how long to wait and how to read it. Always check the leaflet.
Reading it too late. Many tests need reading within a set time, often within 10 minutes. After that, a line can appear that doesn’t mean anything.
Very diluted pee. If you’re testing before your period is due, use your first pee of the day, when it’s most concentrated. From the day your period is due, any time of day is fine.
The test itself. Tests vary in how little hCG they can pick up, so check the box for how early it can be used. Check the expiry date too.
Some medicines. Fertility medicines that contain hCG can cause a positive result. The contraceptive pill won’t affect a test. If you take other medicines, ask a pharmacist.
A recent pregnancy. After a birth, miscarriage or abortion, hCG falls gradually and can take several weeks to clear, so a test can stay positive for a while.

Line tests and digital tests

Both kinds look for hCG, and a digital test isn’t automatically more sensitive. If you’re testing before your period is due, check the leaflet for how early that test can be used, and see early detection tests for more.

Cheap tests and expensive tests

Price doesn’t tell you how sensitive a test is. From the day your period is due, a test that’s in date and used properly should pick up most pregnancies, whatever it costs. Where tests differ is how early they can be used, so if you want to test early, compare what the boxes say rather than the price. You can also get a free test from a sexual health clinic, some GP surgeries and Brook centres if you’re under 25.

What about blood tests?

A home urine test only tells you yes or no. A blood test measures how much hCG there is. You don’t need one to confirm a pregnancy: if your home test is positive, you can refer yourself straight to a midwife. Blood tests are usually used when things are less clear, for example if you’ve had bleeding or pain, and are normally arranged by an early pregnancy unit.

If you’re having fertility treatment

If you’re having fertility treatment, test on the date your clinic gives you (for IVF, this is usually around 16 days after embryo transfer). Testing earlier may give a false result. If you had a ‘trigger’ injection containing hCG, it can stay in your body for up to about 10 days and give a positive that isn’t a pregnancy.

What to do with your result

Positive: it’s almost certainly correct. Here’s what to do next. If the line is very faint, see what does a faint line mean?

Negative, but you still think you might be pregnant: wait a few days and test again. If you get a second negative and your period still hasn’t come, speak to your GP. See false negative pregnancy tests

Questions people ask

Are cheap pregnancy tests as accurate as expensive ones?

From the day your period is due, a test that’s in date and used properly should pick up most pregnancies, whatever it costs. If you’re testing early, check the box for how many days before your period it can be used.

How soon after sex will a test be accurate?

Most tests are designed for the day your period is due. If you don’t know when that is, the NHS says to test at least 21 days after you last had unprotected sex.

Does the time of day matter?

Not from the day your period is due. If you’re testing earlier, your first pee of the day gives the best chance of finding a small amount of hCG.

Can an ectopic pregnancy give a negative test?

It usually gives a positive test, but the NHS says to get advice if you have symptoms of an ectopic pregnancy even if you haven’t had a positive result.

When to get help

Mild cramps that ease with rest are usually nothing to worry about. But if you’re pregnant, or think you could be, get urgent advice from your maternity unit, an early pregnancy unit or NHS 111 if you have:

any bleeding or spotting, even if it’s light
tummy pain that’s severe, on one side, or hasn’t eased after resting for 30 to 60 minutes
pain in the tip of your shoulder

Call 999 if you have bleeding along with severe tummy pain, pain in your shoulder, or feeling sick, faint or dizzy, or if the bleeding is heavy (soaking a pad soon after putting it on). Also call 999 if you have sudden, sharp and intense tummy pain and you feel very dizzy or faint, feel sick or look very pale. These can be signs of an ectopic pregnancy or a miscarriage that need treating straight away.