Lucy Flynn, BABYGO® team
Two lines, and the first thing you did was doubt them. Or it was a faint line, or a line that turned up after you'd put the test down and gone to make a cup of tea, and now you're searching whether a positive pregnancy test can be wrong.
Here's the short version from the NHS, the handful of things that genuinely produce a positive when you're not pregnant, and what to do next in each case.
Quick answer: A false positive is rare. The NHS says a positive test is almost certainly correct if you followed the instructions, and it's the negative that's less reliable. The exceptions are a line read after the time in the leaflet, some medicines, and the pregnancy hormone still in your system after a recent pregnancy, miscarriage or abortion. Ectopic and molar pregnancies give a true positive and need care.
What the NHS says, in short:
- Every test looks for the hormone hCG, which starts to be produced around 6 days after fertilisation.
- Tests are most reliable from the first day of a missed period. If you don't know when that is, test at least 21 days after you last had unprotected sex.
- A positive result is almost certainly correct. A negative is less reliable.
- A result may not be reliable if you don't follow the instructions or test too early, and some medicines can affect it.
- After a miscarriage it can take up to 8 weeks before you have a period again.
Can a positive pregnancy test be wrong?
Rarely, and it helps to know why. A home test reacts to human chorionic gonadotrophin, hCG for short, the hormone your body makes once a fertilised egg implants. The NHS puts it plainly on its doing a pregnancy test page: home tests are accurate as long as you follow the instructions, a positive is almost certainly correct, and a negative is the one to be suspicious of.
So the honest starting point is that two lines usually means what it looks like. The hormone has to come from somewhere.
What people call a false positive is nearly always one of a small number of things, and most of them aren't the test getting it wrong.
What causes a false positive pregnancy test
Here's what actually sits behind the searches, from the everyday to the rare.
- The test was read late. Every leaflet gives a window to read the result in. A line that shows up after that, often greyish and colourless, isn't a result, which is why the NHS puts following the instructions first. Set a timer, read it on time, bin it after.
- Testing too early. That's on the NHS list too, but it usually gives a false negative rather than a false positive: there isn't enough hormone yet. Our piece on how early a Clearblue can detect pregnancy covers the timing.
- A medicine. The NHS says some medicines can affect the result. The leaflet in the box lists the ones that matter for that test, and if you're having fertility treatment, ask your clinic when they want you to test rather than testing early.
- A pregnancy that has just ended. hCG doesn't vanish the day a pregnancy ends. After a miscarriage or an abortion a test can stay positive for a while, which is why the NHS says an abortion clinic may send you a special test to make sure the pregnancy has ended.
- A very early miscarriage. A positive test, then a period a few days later. The test wasn't wrong at the time. It's worth telling your GP if it happens more than once.
- An ectopic or molar pregnancy. Both give a true positive, because there is pregnancy hormone. Neither can continue and both need care, so they belong in this list even though the test did its job. More on the signs below.
| What you're seeing | What it usually means | What to do |
|---|---|---|
| Two clear lines inside the reading time | Pregnant. The NHS says a positive is almost certainly correct | Contact your GP surgery or local maternity unit to book in |
| A faint second line inside the reading time | Still a positive. There's less hormone early on, so the line is lighter | Book in. Test again in a few days if you want to see it darker |
| A grey line that appeared after the time in the leaflet | Not a result. The leaflet's time limit is the whole point | Test again, set a timer, read it on time |
| Positive, then a period a few days later | Possibly a very early miscarriage. The test wasn't wrong when you took it | Tell your GP if it happens again or you're worried |
| Positive weeks after a miscarriage or abortion | Hormone still falling | Follow your clinic's advice on when to test; GP or early pregnancy unit if you're in pain or bleeding heavily |
| Positive with one sided tummy pain, shoulder tip pain or bleeding | Could be ectopic | Contact your GP or call 111 the same day; 999 for the severe signs below |
Two lines and a head full of questions? Tell us your due date and we'll send what to expect, what to do and what to sort each week, starting from now. 10% off when you're ready.
Is it normal to have a positive pregnancy test after a miscarriage?
Yes, for a while. The hormone that gave you the positive rises through early pregnancy and falls once the pregnancy ends, and the fall takes weeks rather than days, longer the further along you were. A test in that window is reading what's left, not a new pregnancy.
The NHS says it can take up to 8 weeks after a miscarriage before you have a period again, so your cycle needs time too. If a test is still positive some weeks on, or you're in pain or bleeding heavily, ring your GP or the early pregnancy unit that looked after you.
The same goes after an abortion: the clinic will tell you when and how to test, and the NHS says they may send you a special test for it.
And if you've had a miscarriage recently and landed here, I'm sorry. The NHS miscarriage pages cover what happens next and where to get support, and none of it needs sorting today.
When a positive test needs a call the same day
An ectopic pregnancy is one where the fertilised egg has implanted outside the womb. The test is positive because there's pregnancy hormone, but the pregnancy can't continue, and it can be dangerous if it's left.
The NHS ectopic pregnancy page lists the signs: a missed period and other signs of pregnancy, tummy pain low down on one side, vaginal bleeding or a brown watery discharge, pain in the tip of your shoulder, and discomfort when peeing or pooing.
If you have those, contact your GP or call 111. The NHS says to call 999 or go to A&E if you get a combination of sharp, sudden and intense tummy pain, feeling very dizzy or fainting, feeling sick, and looking very pale.
A molar pregnancy is rarer. The NHS describes it as a problem with the fertilised egg, so a baby and placenta don't develop the way they should. The hormone level is usually higher than normal, so the test is very much positive, and it's usually picked up at the first scan at around 8 to 14 weeks, or after a miscarriage. The NHS molar pregnancy page has the detail.
What to do after a positive pregnancy test
Believe it, then book it. The NHS says that if you're pregnant and plan to continue, tell them as soon as possible, which in practice means contacting your GP surgery or your local maternity unit to book your first midwife appointment. You don't need a second test to be allowed to do that, though nobody will stop you doing one.
If you'd like the date first, our due date calculator works it out from your last period. And if you tested early and got a negative, that's a different question, answered in how to use a Clearblue pregnancy test and the early detection piece above.
Questions we get asked
Are false positives possible?
Yes, but they're rare, and the NHS says a positive is almost certainly correct. The usual causes are a line read after the leaflet's time, some medicines, or pregnancy hormone still in your system after a recent miscarriage or abortion.
Can a faint line be a false positive?
A faint line that appears inside the reading time is a positive. There's less hormone early on, so the line is lighter, and if you test again in a few days it's usually darker. A faint grey line that only appears after the time in the leaflet doesn't count, and the fix is to test again and read it on time.
Can medication cause a false positive pregnancy test?
The NHS says some medicines can affect the result. Which ones is in the leaflet that comes with the test, and your pharmacist can tell you about anything you take regularly. If you're having fertility treatment, ask the clinic when to test.
How long after a miscarriage will a pregnancy test stay positive?
Weeks rather than days, and longer the further along you were. The NHS says periods can take up to 8 weeks to come back after a miscarriage. If a test is still positive some weeks on, or you're worried, speak to your GP or the early pregnancy unit.
Can testing too early give a false positive?
Not really. The NHS lists testing too early as a reason a result may not be reliable, but the way it goes wrong is a false negative: there isn't enough hormone yet for the test to find. A positive that shows up inside the reading time is a positive, however early you took it.
Once it's a yes, the pregnancy collection is where the small comforts for the next nine months live.
This is general guidance from the BABYGO® team, not medical advice. Your midwife or GP knows your pregnancy; ask them if anything here doesn't fit your situation.
