Everyday Chemistry - How do pregnancy tests 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.

The pregnancy test kits rely on the presence of the glycoprotein hormone, human chorionic gonadotropin (hCG) that is secreted by the placenta in the urine 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. Even though these kits are reliable and accurate most of the times, it is always important to crosscheck with your doctor and go for a check-up in case of any doubts.

CAN you now answer the questions:

What do the pregnancy test kits rely on?

When is test is much more reliable?