False Positive Pregnancy Tests – Why They Can Happen

You may have heard that there’s ‘no such thing as a false positive’… well, this isn’t necessarily accurate!

How does a pregnancy test work?

Once a fertilised egg implants in the lining of your womb, the hormone ‘HCG’ (Human Chorionic Gonadotrophin) is produced. Around the time of your expected period, levels of this hormone should be high enough to be detectable in your urine.

On the test strip, there will be a ‘control’ line which will appear regardless of your result. This assures the user that the test is working and enough urine has come into contact with the test. There will also be a strip of synthetic hormones that will turn pink or blue, depending on the brand of test, when it comes into contact with HCG in your urine.

What is a false positive pregnancy test?

When, on a standard home pregnancy testing kit, another coloured line appears alongside the control line indicating that the user is pregnant. On a digital, a false positive would be a ‘positive’ sign popping up on the screen.

If you’ve done a test, and it’s positive, you are more than likely pregnant so congratulations! However, knowing about false positives can be useful.

What can cause a false positive?

Surprisingly, quite a few conditions, medications and scenarios can cause a false positive! They are very uncommon, thankfully.

  • Polycystic Ovary Syndrome (PCOS) can cause hormonal imbalances, and result in false positives as well as false negatives. I have PCOS and have had so many false positives!
  • Protein and/or blood in the urine from a kidney infection.
  • Rare tumours… but this is extremely uncommon so don’t panic
  • Promethazine (antihistamine)
  • Medicines used to treat Parkinson’s disease
  • Sleeping tablets
  • Tranquillisers
  • Diuretics
  • Anticonvulsants
  • Medicines used for infertility containing HCG.

I had a positive test and then a negative at a later date… was the first a false positive?

It could have been a chemical pregnancy. This happens when a fertilised egg doesn’t implant properly and is lost during your period, so basically a very early miscarriage. They’re actually super common, and we didn’t really know they’d happened to us until the invention of sensitive home pregnancy tests such as First Response Early Response, which can pick up as little as 6.5 MiU of HCG in your urine. Bear in mind that a HCG level below 5 is classed as not pregnant!

It could also have been a dodgy test, in which case you should inform the manufacturer if you have a blood test confirming that you’re not pregnant.

It may have been an evaporation line, which occasionally can seep dye from the control line – evaporation lines are usually grey and colourless, but a coloured evaporation line is absolutely not impossible if a ‘dye run’ has also happened. Reading the test outside the stated time period in the instructions can mean that a line you can see is an evaporation line.

Finally, if the most recent test if a different brand, it may not be as sensitive as the first test you took. Alternatively, if your positive test was taken with first morning urine, and the second was not, this may explain the second test being negative as first morning urine usually contains a higher concentration of HCG.

Good luck to all of our members who are TTC (Trying To Conceive)!


Have YOU ever had a false positive pregnancy test? Tell us your stories in the comments.

Love from Katie. Xx

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