We often see celebrities promoting pregnancy test kits in advertisements on the television. But do you know what these pregnancy tests are and how do they work? Pregnancy tests help in knowing if the woman is actually pregnant or not and are considered as 97 per cent accurate when it comes to results. Let us find out the chemical activity involved in these test kits.

What are pregnancy test kits?

Pregnancy test kits are self-determining diagnostic kits that help in determining if you are pregnant or not. The pregnancy test kit consists of the immunoassay strip which is formed by compressing non-woven fibres into a narrow strip which is then coated with reactive antibodies. These antibodies combine with the pregnancy hormone in a series of steps and ultimately results in a colour change.

The test kit also has an absorbent pad that extends from the test chamber and is used to contact the urine stream. The pad absorbs the liquid and draws it into contact with the immunoassay strip.

Both the test strip and absorbent pad are placed within a two piece housing that allows the unit to be handheld and also protects the strip from environmental contaminants. A leak-proof, clear plastic window on the side of the housing prevents the urine from accidentally splashing on the test strip and also allows the test and control zone portions of the strip to be viewed.

There is also a plastic collection cup which is included with test kits that require collection of urine in a separate step. They may also include plastic vials with pre-measured amounts of reagents that must be mixed with the urine before application to the test strip.

How do pregnancy test kits work?

The pregnancy test kits rely on the presence of the hormone, human chorionic gonadotropin (hCG) in the urine. This hormone is a glycoprotein that is secreted by the placenta shortly after fertilization. The placenta begins developing after the fertilized egg implants in a woman's uterus, which happens about six days after conception. So you can confirm the pregnancy by doing these tests only after six days post-conception.

Fertilization does not always take place the same day as intercourse and so most women are advised to wait until they miss their period before trying a pregnancy test. The hCG levels double about every two days in a pregnant woman, so the test is much more reliable two weeks after conception than one week later.

The tests bind the hCG hormone, from either blood or urine, to an antibody and an indicator. The antibody will only bind to hCG and the other hormones will not give a positive result. The usual indicator is a pigment molecule which is present in a line across a home pregnancy urine test. Highly sensitive tests will use a fluorescent or a radioactive molecule attached to the antibody, but these methods are not used for an over-the-counter diagnostic test.

The tests are available over-the-counter versus obtained those at the doctor's office are the same. The only difference between the two is the decreased chance of user error by a trained technician. Blood tests are pretty much equally sensitive. Urine tests are considered to be the most sensitive and are done by using urine from early morning, which tends to be more concentrated and would have the highest levels of hCG.

Even though these kits are reliable and accurate most of the times, it is always important to cross-check with your doctor and go for a check-up in case of any doubts.