Urine pregnancy test: can it be wrong?

Article originally published on June 30, 2020

Since the invention of urine pregnancy test in the 70s it was possible to know quickly and quite reliably whether you were pregnant or not.

Before the discovery of monoclonal antibodies, which allow the detection of the pregnancy hormone, the well-known hormone hCG, laboratories had much more complex techniques to determine if a pregnancy was on the way: the urine of a potentially pregnant woman was injected into rabbits or female frogs.

If the animal ovulated or laid eggs a few days later, the woman was pregnant. Fortunately, these methods were quickly replaced by others that were less cruel to the animal world.

How does a urine pregnancy test work?

The urine pregnancy test, which can be purchased in pharmacies but also in supermarkets, allows you to do it detect pregnancy hormone in urine. he is 98 or 99% reliability. (depending on the model) from the first expected day of menstruation. Before this date, conventional urine tests were not sensitive enough, but there are so-called “early” devices that can detect the appearance of the hormone hCG in the body a few days earlier.

For a more technical explanation, the test strip you need to pee on contains antibodies that detect the pregnancy hormone. Once the urine is absorbed by the wick, the hormones present are picked up by antibodies, causing a color reaction in the detection band if pregnancy is confirmed (first tier). Antibodies that have not found any hCG to bind to continue on their way and will react with the control band (second band).

What is a false negative?

When it comes to a urine pregnancy test, a false negative is a card that does not show pregnancy when the woman is actually pregnant. It happens, but it’s rare. More often than not, when pregnancy goes undetected, it’s because the test was done too early for the hCG hormone to be detectable.

Urine quality can also be responsible for a false negative : If they are too diluted, the antibodies may not be able to detect the hormone. It is to avoid this problem that manufacturers recommend testing in the morning, with the first urine of the day. Testing during a UTI can also give a false negative.

What is a false positive?

In reverse, a false positive is a test that announces pregnancy when the woman is not pregnant. It’s very rare, but it can happen, especially if you take certain medications such as those used in infertility treatment. The presence of an ovarian cyst can also skew the results, as can abnormal bladder or kidney function.

Finally, a woman can have a miscarriage very early in her pregnancy, which sometimes explains a difference in the results between the urine pregnancy test and the blood test that follows to confirm pregnancy.

How to be sure that the pregnancy test is not wrong?

If the urine test announces an early pregnancy, there is little chance that it is wrong, but to be sure of the result, it is necessary to do a blood test in a medical laboratory to know the precise dosage of the hormone hCG in the blood.

This blood test can be prescribed by a health professional (general practitioner, gynecologist, midwife, etc.) to be reimbursed by the social security, but it is also possible to proceed without a prescription. This test will be the definitive answer to the question “am I pregnant?” “.

If the urine pregnancy test is negative, but the doubt persists, it is entirely possible to do a retest a few days later : if it is negative, there is probably no early pregnancy. If it is positive, the first test was done too early to detect the hormone. A blood test can then be done to confirm the diagnosis.

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