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What causes a False Positive Pregnancy Test?

What causes a False Positive Pregnancy Test?

So, you got that positive result but your period started last week. But, how is that possible? Home pregnancy tests are 100% accurate, right? Now you’re starting to doubt the original result and are sad that it gave the completely wrong answer and has misled you.

This probably won’t make you feel better right now but pregnancy tests done at home are between 97% and 99% accurate. Technically, there was probably nothing wrong with your result and for the test it was positive. But, bear in mind that the test assumes that you will follow the directions perfectly. The tests will check for the level of human chorionic gonadotropin (hGC; the pregnancy hormone) that is in your urine. This is secreted by the placenta once the embryo is implanted and the amount increases considerably over the next week. The tests are able to detect when the level is high enough for you to be pregnant; all women have a small level of the hormone in their body naturally. Positive results show that you have enough of the hormone to be pregnant. So, why have you now started your pregnancy? This is commonly referred to as a false positive or false pregnancy test and there are a number of reasons for this to happen.

A Chemical Pregnancy

These are miscarriages that happen early on. According to experts in fertility, around 70% of pregnancies end with a miscarriage and many women suffer from chemical pregnancies without ever realizing they were pregnancy. The menstrual period happens around the usual time.

However, those who are trying to get pregnant will usually notice that there has been something going on. Many home pregnancy tests can now work just a couple of days after you have ovulated. It is important to remember that while an early positive is exciting, there is the reality of chemical pregnancies.

hCG Shots

When people are receiving help to conceive, they will usually have shots of the pregnancy hormone to stimulate the process of ovulation. It helps with the rupturing of the follicle with the egg and can take up to two weeks to completely leave the system. The level may be enough for the pregnancy test to pick up on to give you that false positive. Before taking a test, make sure you give enough time for the hormone from the shot to disappear: 10,000 IU injections require 14 days, 5,000 IU injections require 10 days and 2,500 IU injections require 7 days.

Evaporation Lines

An evaporation line is a very faint line that appears on a negative pregnancy test after it has dried. All pregnancy tests need to be read within a short time frame after they have been carried out. This is normally about 5 minutes and you should always stick to this time frame.

Pregnancy tests are normally still damp at the time they are read. After a few minutes the urine on the test will start to dry and this drying urine can sometimes cause residue ink in the device to rise to the surface of the test and become darker.

Many women mistake this reading as a positive test. As the urine dries, the line can stay on the strip and reading outside the time frame will give that positive reading falsely.

This is why a vital and rule for obtaining a true positive pregnancy test is: do not read the results after the specified time! The results are only correct in the time frame specified in the instruction leaflet.

Photo Credit: “Dos rayas” (CC BY 2.0) by Daquella manera

This post has been updated since its original publication in 2017.


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