How do home pregnancy tests work?

Today home pregnancy tests are so simple, common, and effective that we take for granted their operation and science behind them. You just have to urinate on a kind of pen, wait a few minutes to know the result and voilà. However, things were not like this a few decades ago.

People used frogs to detect pregnancy

You’re going to laugh, but in the 1930s and into the late 1960s, doctors used an African frog called Xenopus laevis to detect pregnancy. This test was patented by the British doctor Lancelot Hogben and was a common practice, recognized throughout the world. It was about injecting urine samples from the woman under the skin of a female frog and leaving it overnight in her tank.

The next morning the doctor checked for eggs. The human pregnancy hormone initiated the frog’s ovulation.

The science behind home pregnancy tests

The first home pregnancy test came out in 1971, but it didn’t become popular until the 1990s. The basic principle was the detection of the hormone hCG (human chorionic gonadotropin) through a research technique known by the acronym: ELISA (Enzyme-Linked ImmunoSorbent Assay).

The principle is simple and is inspired by the functioning of our own body: foreign agent-antigens enter the body and bind to antibodies, giving rise to an immune response.

This binding is used in the test to detect specific molecules.

On the one hand, the antibody binds to the antigen and on the other to an enzyme, capable of producing a visible reaction that we identify by the change of color.

How do home pregnancy tests work?

Through an animated presentation prepared by Dr. Adriana Sierra and Dr. Aldo Fernando Sosa from the Universidad de las Américas Puebla, this process is explained: “Everything works with a little urine. On the surface of the test is a kind of antibody linked to an enzyme”.

“The antibody is a receptor for the hormone Human Chorionic Gonadotropin (hCDG), which is present in the urine during pregnancy”.

“The structures with hCG-antibodies and the antibodies without hCG travel to the test zone where there are antibodies that are only receptors for the structures that have the hormone Gonadotropin”.

“The interaction with the hormone causes a change of color in the substrate, aroused by the enzyme. The window that is in the test area is tinted. The antibodies without the hCG travel to another control area where there are other antibody receptors, causing a change in the color of the substrate. There’s a baby!”

The line showing the result is not always intensely colored, nor does it occur at the same time, the average time is five minutes after the urine comes into contact with the reactor. However, there are times when a little more time is required. It is important to read the precise instructions for each test.

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When to take a pregnancy test

“Menstrual delay is the first indicator that there may be a pregnancy. There are two early tests that can be done. The first is the dosage of the beta subunit in urine, buying a kit in a pharmacy and the second is the dosage in blood in a chemical laboratory”, says Dr. Rocío González, a specialist in Gynecology and Obstetrics.

Pregnancy tests are based on the detection of the hormone hCG. The end of a test strip is placed directly into the urine stream or dipped into a container where the urine was collected. A few minutes later the test strip reveals the test result. Depending on the test it is one or two lines or even the word pregnant or not pregnant”. “The point is when it is appropriate to do the test.

The sensitivity of pharmacy kits today is very high and with a few days of menstrual delay they already test positive.

But you have to keep in mind that some women can test positive with one day of menstrual delay and others may need a week of menstrual delay to test positive. In contrast, the dosage of the beta subunit in blood made in a medical laboratory is immediate.

The same day of the lack of menstruation can determine if there is a pregnancy in progress”, says the expert.

 Home pregnancy tests are an easy, fast and cheap way to find out if you are expecting a baby. They are available in most pharmacies and supermarkets. Photo: PxFuel
Home pregnancy tests are an easy, fast and cheap way to find out if you are expecting a baby. They are available in most pharmacies and supermarkets. Photo: PxFuel

Don’t crave!

It is very stressful to wonder if you have are pregnant or not, but if you do not want to do a blood test, it is best to wait a bit to do a home test.

For the Mayo Clinic, the probability of obtaining a false negative with home pregnancy tests is greater if you perform it before the second day of the absence of menstruation. If we want the result to be reliable, it is a good idea to wait for the third day, or better yet, a week after the missed period. “Why is it necessary to wait?

Shortly after a fertilized egg attaches to the lining of the uterus (implantation), the placenta forms and the hormone human chorionic gonadotropin (hCG) is produced. This hormone enters the bloodstream and urine.

During early pregnancy, the concentration of this hormone increases rapidly, every two or three days.

The sooner you take your home pregnancy test, the more difficult it will be for the test to detect gonadotropin”, says the Mayo Clinic in the article Home Pregnancy Tests.

You can read: Vaginal discharge during pregnancy, is it normal?

How effective are home pregnancy tests?

Many home pregnancy tests claim to be 99% accurate, or even carry the slogan “before you miss your period”, but that is not entirely accurate, and the reality is that they have different capabilities.

If you take a test and the result is negative, but you think you are pregnant, repeat the test a week later or take a blood test.

“Medications to promote fertility or others that contain human chorionic gonadotropin could interfere with the results of home pregnancy tests. However, most medications, including antibiotics and birth control pills, do not affect accuracy”, Mayo Clinic explains.

The body produces the hormone human chorionic gonadotropin (hCG) only when you are pregnant. Photo: Shutterstock
The body produces the hormone human chorionic gonadotropin (hCG) only when you are pregnant. Photo: Shutterstock

What factors can lead to a wrong negative result?

Until a few years ago, it was believed that false negatives could be less than 1%, but a study by the Washington University School of Medicine in St. Louis revealed that they can be up to 5%. Why does it fail?

  • You get tested too soon.
  • You check the results too soon (read the instructions and if necessary use a timer).
  • You use diluted urine. For best results, perform the test first thing in the morning, when it is most concentrated.

To make sure of the result, it is advisable to do a second test.

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Is it possible to get a positive result and I am not pregnant?

It’s rare, but yes. They are known as false positives.

A false positive could happen if you lost a pregnancy soon after the fertilized egg had attached to the lining of the uterus (biochemical pregnancy) or if you took a pregnancy test too soon after taking a fertility drug containing human chorionic gonadotropin.

“An ectopic pregnancy, menopause, or problems with the ovaries could also contribute to misleading test results”, Mayo Clinic explains.

If your result is positive, see your doctor as soon as possible to start prenatal treatment and, if your result is negative, but you still do not have your menstrual period (amenorrhea), in addition to repeating the test or doing a blood test, it is important that your doctor evaluates other possible factors such as thyroid disorders, low body weight, ovarian problems, excessive exercise, or stress.

Remember that your health care provider can help you regulate your menstrual cycle.

Translated by: Ligia M. Oliver Manrique de Lara

Spanish version: Here

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