How does your pregnancy test know you are pregnant?
By: Eugenia Ibarra
Tags: Health, Baby, Pregnancy Test, Medical, Pregnancy

Got a positive pregnancy test? Not sure? What if you got a negative but you are certain to be pregnant? Well, though pregnancy tests tend to be very accurate they do have mistakes. But in order to understand that you must first understand how pregnancy tests work, and know that they work through hormone detection in your urine. During your whole menstrual cycle there are different hormones that control it. But during pregnancy the concentration of the hormones can either increase or decrease, to maintain and help the pregnancy develop.

T he main pregnancy hormone, or well the one to look for when wanting determine if you are pregnant or not is the Human Chorionic Gonadotropin HCG, which is created in the trophoblast tissue (found in early developing embryos), it is also released from the pituitary gland in the brain. The HCG has two main jobs: it tells the body NOT to menstruate (not to shed your uterus lining) because you are pregnant, and later on it contributes to the formation of the placenta which feeds the embryo.

At home pregnancy tests use different zones and chemical reactions that react differently depending if there is a presence or absence of HCG. In the test, the urine travels from the sample area of the test to where some special proteins (antibodies) and enzymes bind to the HCG to carry it to the next zomes. AB 1 enzyme and AB 2 enzyme “sandwich” the HCG in order to stick to the test zone and produce a chemical reaction that will show the first line that usually appears. The second line is always present to show that the pregnancy test works, it is created in the control zone where the lone antibodies go and bind to immobilized antibodies. A similar chemical reaction is created and a second line is shown. That’s why double lines or a cross means pregnant, since there were two reactions, and one line or a minus line means not pregnant, since the only reaction was in the control zone with lone antibodies.

Are these tests reliable?

It is estimated that approximately 90% of the time the test is accurate, though there’s of course that chance that the test is wrong. First of all the test might be faulty and not work properly or not work at all, in which case the second line (not pregnant line) will not appear at all. When you receive a negative result but you suspect that you are pregnant it is called a False negative and they can happen when HCG concentrations are very low, since during the first days or even week the amount is not enough to give you a positive result, but the HCG concentrations double each 2–4 days, so trying again later might work. Another reason why you might get a false negative when your urine is diluted, this means that you have drank a lot of water or other drinks, which is why it is best to take the pregnancy test first thing in the morning.

On the other hand you might receive a positive result and not be pregnant at all, this is known as a False positive. These results are more rare than false negatives and only occur in specific situations like for example if you are on IVF injections or any other type of fertility drugs, might have uterine or ovary cancer, have menopause or have an ectopic pregnancy (pregnancy outside the uterus).

What to do next?

Consider waiting a week or two and take another test, of course treat your body as if you are pregnant, since even the smallest amounts of drugs, alcohol, tobacco or other restrictions might severely harm the embryo, even if you are not aware of its existence. After the second, third or fourth test you should go to the doctor to confirm your results through their own tests which are more accurate since they can detect the exact amount of HCG in your body, so you can know how far along you are or even if the HCG is produced by another factor (though high amounts of HCG outside of pregnancy are rare). As you can see, even if you suspect you have a false positive it is very important to consult a doctor, as the causes of the false positive are also significant to your health and need to be treated. And even if you have several false negative tests and you still have suspicions that you are pregnant, consult a doctor to make sure. So in conclusión, take several tests since depending on the week of gestation the amount of HCG might vary, do it first thing in the morning to avoid diluted urine giving you false negatives and after you´ve taken several tests visit the doctor in order to take the next step properly, never do anything that can harm you without medical advice.

Betz, D., & Fane, K. (2020, August 30). Human Chorionic Gonadotropin. Nih.Gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK532950/

Compound Intrest. (2018, November 9). How do pregnancy tests work? Compound Interest. https://www.compoundchem.com/2018/11/09/pregnancy-tests/

MayoClinic. (2019). Home pregnancy tests: Can you trust the results? Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/getting-pregnant/in-depth/home-pregnancy-tests/art-20047940

‌TED-Ed. (2015). How do pregnancy tests work? — Tien Nguyen [YouTube Video]. In YouTube. https://www.youtube.com/watch?v=aOfWTscU8YM