Your pregnancy test might be showing a false positive, here are 8 reasons why (2024)

At-home pregnancy tests are pretty damn accurate, right? If you get a positive result you can almost certainly assume you are pregnant, at which point you should visit your GP to confirm the news, and then you can spend the next nine or so months preparing for your human child, alongside these celebrities who are also currently expecting.

But can a pregnancy test be wrong? On some rare (and we mean very rare) occasions, your test might display a false result. This could either be in the form of a 'false positive' - a test that says you're pregnant when you don't actually have a viable pregnancy, or it could be in the form of a 'false negative' - a negative result when you are, in fact, pregnant.

We spoke to Dr Preethi Daniel, medical director at London Doctors Clinic, to talk us through all the ways either of these eventualities could happen.

But before we start, Dr Preethi is keen to reiterate that "home pregnancy tests have advanced considerably in the technology they use and, with rigorous testing, have increased in accuracy over the years."

She does suggest that if you want to make sure you've got the absolute best chance of accuracy, incorporate the test into your morning routine. "Catching your first urine of the day ensures HCG levels are at their highest," she said.

Keep reading to find out whether a pregnancy test can be wrong, and further, why.

Can a pregnancy test be wrong?

In short: yes. And there are several reasons why, ranging from the test having gone past its expiry date (yep, they don't last forever!) to certain kinds of medication. There are sadder reasons for a pregnancy test to be inaccurate too, including miscarriage (which is thought to impact around 1 in 8 known pregnancies, as per the NHS). We've explained all the reasons in more detail below.

1. Your pregnancy test has expired

"Pregnancy tests expire, which can result in a false-negative test. Heat or moisture can cause damage to the test window on a pregnancy test and this can also result in inaccurate results. If you want to buy a pregnancy test, go to the pharmacy so you can guarantee it's within date," explains Dr Preethi.

2. You're on fertility medication

"HCG (or if you want to get technical, human chorionic gonadotrophin) is a hormone released by the placenta after the embryo has been implanted into the womb. This hormone can be detected in the woman’s urine or blood even before a missed period. But if you've been going through fertility treatment, like IVF for example, and you are given fertility medication, this can cause a false positive reading on a pregnancy test. Fertility treatments are hormones which may mimic or even contain HCG, which is why this may happen."

3. Miscarriage

"If you have had a recent miscarriage, the HCG level can stay in your bloodstream for several days. This can result in a false positive test result."

4. Medication

We're pretty sure we've answered your question to 'can a pregnancy test be wrong?' by now, but if not, fun fact: even your medication can meddle with your result.

"Some medications can alter the level of HCG in the blood. These include benzodiazepines taken for anxiety (diazepam, alprazolam), diuretics taken for excess water retention (furosemide) and even antihistamines such as promethazine.

"These medications could theoretically lead to a false positive result if a woman has experienced either a chemical pregnancy (more on that below) or a miscarriage, both of which would leave a small, undetectable level of HCG in the woman's bloodstream. The medication could increase this level slightly, bringing it into a detectable range, hence the possibility of a false positive result emerging from the test."

5. Chemical pregnancy

"This is where a sperm and an egg meet but the fertilised egg - known as an embryo - is unable to implant in the womb. This can still cause a rise in HCG levels despite there being no viable pregnancy. For some women, this false positive result can be emotionally draining. For this reason, it is always recommended you wait a week or so after you miss your period to take a pregnancy test," Dr Preethi continues.

6. Ectopic pregnancy

"An ectopic pregnancy can result in a positive reading on a pregnancy test, and this does indeed mean you are pregnant. Sadly, though, this pregnancy is not in the right place. The foetus is most likely in one of your fallopian tubes (the tubes that carry the egg to the womb) and this is a medical emergency. If you have a positive test result at home and start experiencing abdominal pain, bleeding or feel unwell, you must see a doctor as soon as possible."

7. You've left the test sitting too long before looking

"You've urinated on the stick, and are now eagerly awaiting. You might want to take your mind off the nervous wait, but don't wander off and forget; leaving the test 'cooking' for too long can give a false positive result. The reason for this is due to the urine evaporating if left for too long; it can leave a faint line which can be mistaken as a positive test. It is recommended to not read a pregnancy test after the recommended time frame (10 minutes) as most brands have a possibility of evaporation lines."

8. Medical conditions

"There are certain medical conditions that can impact the outcome of a pregnancy test, explains Dr Preethi. If you're suffering from a urinary tract infection, kidney disease or have ovarian cysts, this may alter the result of your test. How? Well:

• Urinary tract infection or kidney disease may result in a contaminated sample (with red or white blood cells in your urine) which may result in a false positive result. It does depend on the method used by individual brands but in general, any impurity or even blood can cause a false reading.
• Ovarian cysts, cancer or disorders of the pituitary gland (in the brain) can either produce HCG leading to a false result or produce hormones mimicking HCG, once again leading to a false positive. These cancers often produce small amounts of the hormone, but home pregnancy tests can be sensitive enough to pick this up.

So there you have it. While it's very, very rare, pregnancy tests can indeed show false positives. If you have any doubts, make sure to book an appointment with your GP for further advice.

This article is not intended to be a substitute for professional medical advice or diagnosis. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.