How Pregnancy Tests Work

Karla asks: How do pregnancy tests work?

Pregnancy-testsAh, the pregnancy test. Karla, I can only hope the reason you might be wondering is you’re taking one in a joyous scenario, and you’re trying to have a child, rather than the other way with a little more sweat on your forehead! 😉

Either way, pregnancy tests that use urine samples have shockingly been around since 1350 BC. Yes 1350 BC! (See Bonus Facts below for more information on this ancient, surprisingly accurate test.) However, modern pregnancy tests didn’t began to hit the market until 1978. They rely on an antibody for a hormone known as human chorionic gonadotropin (hCG). If the tests detect the presence of this hormone, they react and give you a positive sign. How does this happen? I’m glad you asked because otherwise this article would have been too short and lacking in detail!

When a fertilized egg implants in a woman’s uterus, the placenta begins to develop. Specialized cells within the placenta begin to secrete hCG- specifically, trophoblast cells. This process usually begins about 6 days after implantation of the egg. This is why you need to wait about 6 days after intercourse before you will have a chance of testing positive.

A pregnancy test uses an antibody known as an immunoassay to detect the presence of hCG. The makers of the test then synthesize another antibody and label it with a dye or an agent that produces light as a result of a chemical reaction. They combine these two antibodies based on the “sandwich principal”. When the antibody that binds to hCG attaches to the hormone, the second antibody also attaches and releases its hold on whatever molecule allows for visual recognition. The result is a visual marker that you can see… Science!  Congratulations, you are pregnant.  If the antibody doesn’t attach, the second antibody will not release the visual marker and nothing is seen.  You’re not pregnant.

Once a woman’s placenta begins to secrete hCG, it doubles the amount it produces about every 48 hours. It continues to do this until its peak around the 11th and 12th week of gestation. After this, it begins to decline and then secretes low levels throughout the rest of your pregnancy. This is why most tests (and doctors) tell you to take the test after your first missed period. If you follow this advice, they are between 97-99% accurate.

If you’re worried that your negative test result is not as accurate as the tests done at the doctor’s office, don’t. They’re the same test. The biggest difference is your doctor might perform a blood test to detect hCG as well, rather than or in addition to testing urine. While hCG levels are nearly the same in urine as they are in blood, there are some situations where you can have urine levels of hCG and not have it in the blood. These are the most common reasons for a false positive.  So pee confidently Karla! Modern science has made telling if you are pregnant amazingly accurate and easy.

If you liked this article and the Bonus Facts below, you might also like:

Bonus Facts:

  • The earliest written record of urine being used to detect pregnancy came from Egypt. Dated around 1350 BC, this Egyptian method had woman thought to be pregnant, pee on wheat and barley seeds over several days. They stated if the barley grew, the woman was pregnant with a male child. If the wheat grew, a female child was incubating. Now, this might sound like voodoo medicine, but in 1963 this theory was tested and found to have some validity. About 70% of the time, the urine of pregnant women promoted seed growth, while non-pregnant women and men’s urine did not. The theory was that elevated levels of estrogen was the key to the seed growth.
  • Most cells in our bodies have processes that are regulated by hormones. Hormones generally belong to 4 different groups: peptides or proteins, steroids, amino acid derivatives, and fatty acid derivatives. hCG is a glycoprotein hormone.
  • The first test to use urine levels of hCG to detect pregnancy was developed in 1927. Known as the A-Z test, this test involved taking a woman’s urine and injected it into a young rat or mouse that wasn’t old enough to procreate. If the urine had hCG in it, the rodent would sometimes show signs of estrous (in heat).  These tests were not very accurate.
  • The first modern day home pregnancy test was marketed by e.p.t. It was a bit more complicated than the “pee-on stick” versions we have today. It consisted of a vial of purified water, a test tube with red blood cells from a sheep, a medicine dropper, a clear plastic support for the test tube, an angled mirror at the bottom, and some duct tape (OK, I made the duct tape part up). Taking quite a bit longer to get results (around 2 hours), it was still 97% accurate for positive results and 80% accurate for negative results. The first advertisement for the test came in April 1978 edition of “Mademoiselle”. The advantages stated were, “privacy and not having to wait several more weeks for the doctor’s confirmation, which gives you a chance, if pregnant, to start taking care of yourself… or to consider the possibility of early abortion.”
  • Testing using immunoassays is not just for home pregnancy tests. They are also used in several other ways like: testing for diseases such as HIV and hepatitis, urine tests for illegal drugs, and testing the therapeutic levels of other drugs to name a few. In fact, there are over 70 different ways modern science is using immunoassays, including in fields such as agriculture, environmental, food, industrial, water quality and veterinary care. It has become such a popular method of testing, it has grown to a $6 billion a year industry with over 2.5 billion tests sold annually worldwide.
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  • This is a great article and I loved reading it! However, there is one small error:

    “This process usually begins about 6 days after implantation of the egg. This is why you need to wait about 6 days after intercourse before you will have a chance of testing positive.”

    Intercourse that leads to conception occurs around the time of ovulation, whereas implantation does not occur until one to two weeks after ovulation. The quote above sounds like the two happen at the same time.

    But in any case, I enjoy these posts a lot, thanks for writing them!

  • Are all pregnancy tests deemed equal in accuracy then? if so what’s the point in going out and purchasing a $20 test when i can go to the dollar store or CVS and buy one for $5? (hypothetically of course, idk what the actual prices of these are)