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If you think you’re pregnant, and you want to confirm it, it’s normal for you to go to a pregnancy test. Almost always what we usually choose is to compare these tests in the pharmacy, which are very easy to perform at home with a urine sample. Most of these pregnancy tests claim to be 99% effective, but there is that 1%. And they are not infallible and there are factors that can provoke both false positive and false negative.

In this article we will explain what is a false positive, and what are the most common causes for it to occur.

What is a false positive and how do pregnancy tests work?

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Pregnancy tests work by detecting the human chorionic gonadotropin hormone (hCG), which is usually only present in the woman’s body if she is pregnant. If the pregnancy test is positive, it means, almost certainly, that you are pregnant or that you have been pregnant.

To get a real false positive, something that is quite rare, only happens if you have hCG in your body for another reason other than a pregnant woman. These reasons may be, because you have recently been pregnant, if you are taking hCG fertility drugs or because of a state of health, such as some rare ovarian cysts. A pregnancy test can also give you a false positive if you have a chemical pregnancy, an early miscarriage, an ectopic pregnancy or a molar pregnancy, but this is not exactly considered a false positive.

There may also be a false negative. This is that even if you’re pregnant, hCG levels are still too low for the test to be detected.

Reading error due to evaporation in urine

When you have a pregnancy test at home it is very important that you follow the instructions to the letter. If you have changed your test brand read those of this new brand, because each one has its instructions.

Almost all pregnancy tests ask you to read the results within 4-5 minutes after they are performed. Don’t take more than 10 minutes, and never get around 30. This is very important because most non-digital urine pregnancy tests give one line if you are not pregnant and two lines if you are pregnant, or a positive or negative sign appears. But if the pregnancy test is read a long time after the recommended time, it may happen that another line appears from the evaporation of urine.

So, you might interpret a false positive. This second line that appears is not a test that you are pregnant, but an evaporation line that leaves urine after the recommended time to read the test.

False positive for medications or certain diseases

Certain medications affect the results of pregnancy tests. As we have said the level of the hormone hCG is measured, so drugs that include this hormone as an active ingredient, are usually those used to treat infertility, will alter measurements. It can give you a false positive when you’re not pregnant yet.

There are also some diseases and medical conditions that can increase a woman’s hCG, even if you’re not pregnant. These are disorders that affect the pituitary gland and hormone levels, we talk about perimenopausal or menopausal women; gestational trophoblastic diseases, cancers of the ovary, bladder, kidney, liver, lung, colon, breast and stomach.

Other reasons for false positives are molar pregnancies, this is when a sperm fertilizes an empty egg; a recent pregnancy, with birth, or with miscarriage or induced. They are actually real positive pregnancy results even though this does not continue. Another reason is ectopic pregnancies, something rather rare.

So don’t be alarmed about all this, because we repeat you again, the most common thing is that if the pregnancy test has tested you positive, and you’ve done well, is that you’re pregnant.