How do pregnancy tests work?

Every pregnancy test, whether it’s a cheap strip or a digital one, works the same way: it looks for a hormone called human chorionic gonadotrophin (hCG) in your wee.

Where hCG comes from

hCG is made by the cells of the developing embryo that go on to form the placenta. It starts once the embryo implants in your womb, about 6 to 12 days after ovulation, and it can be found from about a week after conception. From there the amount builds up quickly. Read more in hCG: what it is and how it works.

What happens inside the test

The tip of the test soaks up your wee and carries it along a strip. The strip contains antibodies that stick to hCG and carry a coloured dye. Further along, a band of fixed antibodies catches any hCG that has dye attached, so a coloured line or symbol appears in the result window. A digital test reads the strip for you and shows words instead.

Most line tests also have a control line, which should appear every time, pregnant or not. It shows your wee has travelled along the strip and the test is working. If it doesn’t appear, the result doesn’t count and you’ll need to use a new test.

Why some tests are more sensitive

Tests vary in how little hCG they can pick up. The sensitivity is sometimes shown on the box in mIU/ml: one brand’s early test detects 10 mIU/ml, while the urine test used by one NHS hospital lab detects 25 mIU/ml. A more sensitive test may show a positive a little sooner. See hCG levels and test sensitivity.

Home tests and blood tests

Home tests only tell you whether there’s hCG in your wee. A blood test at a hospital or clinic can measure exactly how much there is in your blood. You don’t need one to confirm a pregnancy, but an early pregnancy unit may use them, for example if you have pain or bleeding, often taking two tests 48 hours apart to see how the level is changing.

Why timing matters

A test can only show a positive once there’s enough hCG in your wee for it to detect.

Some early tests can be used from 5 days before your period is due (the packet may say ‘6 days before your missed period’), but they won’t pick up every pregnancy that early. Testing from the day your period is due is more reliable, though a negative can still be wrong if there isn’t enough hCG yet. If your period doesn’t come, test again a few days later.

See how early can I take a pregnancy test? for what to expect on each day.

Doing the test

Always follow the leaflet, as tests vary. Our guide to the best time of day to test, and how to do one goes through it step by step, including how to read the result.

Why a result can be wrong

A positive result is almost certainly correct. That’s because the test only shows a result line when hCG is there to be caught, and hCG is very rarely present unless you’re pregnant or have recently been pregnant.

A negative result is less reliable. It may be wrong if you test too early, your wee is very diluted, or you don’t follow the instructions. Very rarely, extremely high levels of hCG can stop the antibodies working properly and give a negative. This is called the hook effect.

A positive can occasionally happen without an ongoing pregnancy:

after a fertility medicine that contains hCG
after a recent pregnancy, as hCG can stay in your body for several weeks after a birth, miscarriage or termination
rarely, because of some medical conditions

See can a pregnancy test be wrong? for more.

After your result

Positive: let your GP or local maternity unit know, so you can start your antenatal care. See tested positive? What to do now.

Negative: if you still think you might be pregnant, wait a few days and test again. Speak to your GP if you get a negative result after a second test and your period still hasn’t come.

Questions people ask

Do cheap pregnancy tests work as well as expensive ones?

They all look for the same hormone. They differ in how small an amount they can detect, which matters most if you test early. From the day your period is due, a basic test is usually enough.

Can a pregnancy test detect an ectopic pregnancy?

A test can be positive with an ectopic pregnancy, but it can’t show where the pregnancy is, and ectopic pregnancies often make lower levels of hCG. Get advice if you have symptoms of an ectopic pregnancy and could be pregnant, even if you haven’t had a positive test.

Can medicines affect a pregnancy test?

Some can. Fertility medicines that contain hCG are the main ones: if you’re having fertility treatment, test on the date your clinic gives you. Tell your GP or pharmacist about any medicines if a result doesn’t make sense. See can you get a false positive?

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.