How to interpret a pregnancy test: are there any false positives?

For anyone taking a pregnancy test, the results are bound to be important, and in some cases, life changing. Whatever your personal reason for taking a test, you need to know that you can interpret the result correctly and that you can rely on its accuracy. So how accurate are the sorts of tests that you take at home? Can you get a false positive pregnancy test, when might you have reason to doubt the result and what should you do in that case?

In a nutshell, a false positive pregnancy test is very unlikely but not impossible under some circumstances. A false negative is also quite unlikely but it is a lot more common than a false positive. We will look at the reasons for this difference, and start out by explaining exactly how the pregnancy test works.

How does a pregnancy test work?

All home tests work in the same way, although the method of use and the way they display the result can differ. About six days after you conceive, the embryo attaches itself to the lining of your womb. This is known as implantation. At this point your body starts to produce human chorionic gonadotropin (hCG), known as the pregnancy hormone, and the levels increase quite quickly, doubling in volume every two to three days. This hormone is detectable in both urine and blood, and it is the presence of hCG that the pregnancy test identifies.

A blood test taken in a doctor’s office is more sensitive because it´s able to detect very low hormone values and can identify the presence of hCG earlier than a home kit, as early as six to eight days after ovulation, but for most purposes it is more likely that you will be testing at home with an over-the-counter test kit. Some kits require placing a testing stick in your stream of urine, others need you to collect some urine in a container and dip the stick in, or you may need to collect some urine and draw up a drop in a tube to place a drop on to the test stick. Whatever the method, results will show in one of several ways:

• There could be a plus or minus sign. Interpret this as plus for positive (i.e. you are pregnant) and minus for negative (you are not pregnant).
• The result may spell out the simple words ‘not pregnant’ or ‘pregnant’.
• It could be a single line for negative or a double line for positive.

How common is a false negative pregnancy test?

Most brands claim that their tests are 99% accurate as long as the instructions are followed and the test is taken at the right time. False negatives, that is, a test result which shows you are not pregnant when in fact you are, are more common than false positives for a few reasons. You could have taken the test too early, before sufficient levels of hCG in urine had built up. You may have had too much liquid to drink before testing, making your urine is too diluted. The result can also be affected by some medications such as antihistamines or diuretics.

Can you get a false positive pregnancy test?

We have already seen that a false positive pregnancy test is highly unlikely. If there is a positive signal on the test result, even if it is very faint, the overwhelming chances are that you are pregnant. In the rare cases that a false positive does occur, there are a number of possible reasons.

• If you left the testing stick too long before trying to read the result, there could be evaporation lines remaining from the urine drying out and it’s quite possible to mistake this for a positive line. To avoid this, make sure to read the result after the recommended time, which is usually around five minutes and certainly no longer than 10 to 30 minutes later. To find out more about interpreting the result, you may like to read our blog article about a faint line on a pregnancy test.
• If you have recently had a miscarriage or an abortion, it is possible that the levels of hCG, which decline slowly after losing a pregnancy, have not reduced enough to register a negative result. If you have any doubts for this reason, it’s best to wait at least 19 days, which is the average time it takes for hCG to fall back to non-pregnant levels, and then test again.
• A molar pregnancy is a condition caused by genetic abnormalities which happen at conception and lead to a non-viable pregnancy. This can take the form of a growth resembling a bunch of grapes, as the growth consists of small fluid-filled sacs. The condition needs to be treated with a dilation and curettage and after this surgery the woman’s hCG levels need to be monitored carefully to make sure that all the material has been removed. If any remains, it would not only cause a false positive pregnancy test but could also lead to further serious complications.
• There are also some medications which could be the cause of a false positive pregnancy test. Most notably this would be if you were undergoing fertility treatment and had medication which contains hCG as an active ingredient. However, in these circumstances you would be under medical supervision and more likely to have a blood test for pregnancy at the appropriate time.

Seeking more information, advice and help

We know that any time you are taking a pregnancy test is potentially stressful and worrying. Rest assured that in almost all cases, as long as you follow the instructions, the result you get will be correct, and we hope it turns out to be the one that you wanted! If you have any lingering concerns about pregnancy or fertility and would like to find out more about assisted reproduction, do get in touch with us at IVI. You can watch the video introducing who we are and what we offer, ask for more information or simply go ahead and make an appointment using our online contact form.