Why pregnancy tests can show the wrong result

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Pregnancy tests are now so advanced that most brands can confirm pregnancy five days before a woman is due to get her period. But sometimes, even these modern tests can get it wrong.  

They're all designed to detect a hormone called human chorionic-gonadotropin (hCG), which the body starts to produce once an egg has been implanted. The hCG shows up in your urine, and there are two different types of tests you can take: qualitative and quantitative.

A qualitative test picks up on units of hCG above 25 (measured in mIU per millitre). It can only determine if hCG is in your urine – it can't give you a reading of the level of hCG. Pregnancy tests that show two lines, a happy face or the words 'yes' or 'pregnant' all fall into this category.

A quantitative pregnancy test, however, will not only confirm that you're pregnant, but also how many weeks along you are. Blood tests are the best way to measure the exact amount of hCG in the body, but there are also some digital home pregnancy tests that can give you an estimate of how many weeks you are (these will display either 1, 2 or 3+ weeks in the window).

Although only a quantitative blood test can confirm a pregnancy completely, using a qualitative urine test at home will probably give you a correct result. However, there are some situations where you can get a false positive or false negative.

Why you might get a false negative

1) Testing too soon

Being able to test so early nowadays doesn't always mean you'll get an accurate result on the first day you can test. The chance of getting a correct result increases the closer you are to your period due date, so if you test earlier than that date, you might get a negative - even if you are pregnant.

Implantation usually takes place six days after an egg has been fertilised, but it depends on when conception actually happened, and your cycle length.

If you're actively trying for a baby you might be tempted to test as early as possible, but it's better to wait until the day your period is due, or to buy more than one test so you can check every few days.

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2) Not following the instructions

All brands of pregnancy tests have different guidelines when it comes to the amount of time you need to pee on the stick or dip it in the cup of urine, and how long you need to wait before reading the result. If any of the steps are carried out incorrectly, if urine splashes on the results window, or the test isn't laid on a flat surface, the results can be affected. Take the time to read through the instructions first.

3) Diluted urine

If you take a test shortly after you've drunk a lot of water, your urine could be too diluted to pick up enough hCG. It's best to do a pregnancy test in the morning when your urine is most concentrated.

4) Testing late in pregnancy

This is very rare, but sometimes if a woman takes a test at the end of her first trimester or later, the test might not detect hCG, as her levels may be higher than the test is designed to pick up.

This is called "the hook effect" and could explain why some women believe they're not pregnant until they give birth, even if they do take a test.

Why you might get false positive

1) Chemical pregnancy

A positive test doesn't always mean a pregnancy is viable. A miscarriage can occur so soon after implantation that it may not even delay your period, and you might not have noticed you were pregnant except for your positive test result. This is called a chemical pregnancy and it's very common.

Follow-up blood tests will be able to tell whether your hormone levels are dropping or rising.

2) Evaporation lines

Sometimes an 'evaporation line' can appear on a test as urine dries in the test window, and some women may confuse it for a very faint second line. It's most likely to show up when a test is viewed after the recommended reading time has passed, and it can be a different colour to the line that will appear with a positive result. You should take another test if this happens.

3) Prescription medications

Some medications contain hCG that can be picked up on a pregnancy test, even when you're not pregnant. These include hCG injections for fertility treatment, medications for convulsions and Parkinson's disease, some tranquilisers and diuretics, and even certain types of antihistamines. If you're taking any of these you might be warned to wait until the medications are out of your system before taking a test.

4) Underlying medical problems

Certain medical conditions can affect pregnancy test results, whether or not you're been diagnosed before falling pregnant. For example, if you have a UTI or kidney problems, blood or protein might be in your urine, which can change the test result. Ovarian cysts, menopause and antibodies can also affect hormone levels.

It could also be an issue that develops between ovulation and implantation, such as an ectopic pregnancy or a molar pregnancy. An ectopic pregnancy is when an egg implants somewhere besides your uterus, usually in the Fallopian tubes, and a molar pregnancy is when a fertilised egg grows into a tumour instead of a baby, somewhere in the tubes or uterus.

With an ectopic pregnancy, hCG will be produced but it will be unusually low because the egg won't be growing as it normally would. With a molar pregnancy, hCG levels may be unusually high because the tissue grows more quickly than in a normal pregnancy.

This is why it's important to see your GP for a follow up blood test after getting a positive result on a home test: your doctor will be able to check if your hCG levels are rising and whether they're within normal range.

5) Previous miscarriage or abortion

If you recently miscarried or had an abortion there might still be hCG in your body from the previous pregnancy. Women who are hoping to fall pregnant again soon after a miscarriage may be disappointed to realise that a positive test doesn't necessarily mean they are pregnant again, as it can take a while for hormones to return to their pre-pregnancy levels.

Other factors that can create false positives and negatives

1) A dirty cup

If you collect your urine in a cup you've used for a test before - if the cup still has traces of old urine, soap or water - the results can be affected. Use a new cup or buy a pee stick test instead.

2) Expired or faulty test

Always check the use-by date on a test. An expired test could give an inaccurate result if the dyes that are used for the test and control lines have faded, or if the chemicals are no longer strong enough to detect hCG.

A test can also be faulty if it has been stored in a hot or moist area (such as the bathroom), in an open box.