An over-the-counter pregnancy test is usually the first port of call to check if you could be pregnant, and home tests are around 99 per cent accurate.
However, if you’ve got too much of a certain hormone in your body, you could still be pregnant even if your test appears negative.
How does this happen?
It’s called the ‘hook effect’ and it means you could be seeing a negative result on a pregnancy test even when you are pregnant.
The hook effect occurs when a woman has irregularly high levels of a hormone that is produced during pregnancy.
Pregnancy tests are designed to detect a hormone in your urine called human chorionic gonadotropin (hCG).
This hormone is produced after a fertilised egg attaches itself to the wall of the uterus.
If you are pregnant, your levels of hCG will increase rapidly, doubling every two to three days.
What’s the hook effect?
The hook effect intervenes with the signals in the body that indicate pregnancy.
When you’re pregnant, the hCG molecules would get sandwiched between two antibodies and this is how it’s detected by a pregnancy test.
However, if you’ve already got too much hCG in your body, the antibodies will be flooded and the test will come back negative. This is called the ‘hook effect’.
It’s more likely to occur in cases of twins or triplets, because the level of pregnancy hormone is so much higher.
Other reasons for a false negative
The hook effect in itself is pretty rare, but there are other reasons for producing a false negative.
The most common reason for a false negative is testing too early.
According to the NHS, “pregnancy tests are most reliable from the first day of your period” although different tests can vary from when they can be used, so it’s always best to check.
If your periods are irregular and you’re not sure when you’re due, the rule of thumb is to wait at least three weeks after you think you might have got pregnant before taking a test.
Another reason for experiencing a false negative is your urine being too diluted.
It’s best to take a test in the morning before you’ve drunk anything as this is when your urine is at its most concentrated.
How to know for certain if you’re pregnant
As previously stated, home pregnancy tests are usually highly reliable, with around 99 per cent accuracy.
But with extra variables factored in, there can be false positives and false negatives when testing at home.
If you’ve done home tests but are still unsure of whether you’re pregnant, make an appointment with your GP.
Your GP can arrange a blood test to check if you’re pregnant.
The results might take longer, but they can detect pregnancy at an earlier stage than home urine tests.
This article originally appeared on our sister site Edinburgh Evening News