That little plastic stick looks so innocuous doesn’t it, yet it wields a great power – the power to change your life and the people in it…forever!!  Have you ever stopped to wonder just what pregnancy tests are and how on earth they work?

Here, BBI Solutions explains what it is that makes the humble yet powerful pregnancy test work:

What are pregnancy tests and what makes them work?

A pregnancy test determines whether a woman is pregnant. There are certain markers that can indicate if a woman is pregnant which can be found in urine and blood, and pregnancy tests are able to detect these specific markers through an amazing process.

The main marker a pregnancy test is looking for is something called hCG (Human Chorionic Gonadotropin), which is produced by the trophobloast cells of the fertilised egg. hCG is the most common marker that pregnancy tests are able to find and use to determine a pregnancy.

Although, whilst hCG is a reliable marker of pregnancy, it cannot be detected until after implantation of a fertilised egg. This can sometimes result in false negatives if the test of performed during the very early stages of pregnancy.

Positive or negative…?

Sometimes, getting a simple positive or negative, isn’t as easy as it should be! A number of factors can mess with the outcome of a pregnancy test. Improper usage of home testing pregnancy kits can results in both false negative and false positives due to misunderstanding or failing to follow the instructions included in the kits. How hard is it to pee on a stick you’re wondering, right?

When used by experienced technicians, home pregnancy tests are almost as accurate as professional laboratory testing with 97.4% accuracy, however when used by consumers, accuracy falls to 75%.  False negative outcomes can occur when testing is done too early in the stages of pregnancy. Most urine tests begin to detect hCG levels shortly after implantation, which can occur anywhere from 8 to 10 days after ovulation. The levels of hCG found in urine rise throughout the first 20 weeks of pregnancy, so accurate readings increase as time goes on.

It can take up to 12 further days for implantation to happen which can mean even the most sensitive pregnancy tests may give false negatives up to 17 days after intercourse.

False negatives results may occur for several reasons, including mistakes made when testing, use of drugs containing the hCG hormone and non-pregnant production of the hCG molecule.

So, how do pregnancy tests work?

A home pregnancy is a lateral flow test, also known as a “Sandwich ELISA” (ELISA stands for Enzyme-Linked Immunoabsorbant Assay) this means that the pregnancy test depends on a fluid or substance to work out if you are pregnant or not.

The job of a lateral flow test is to detect (and sometime quantify) the presence of a certain marker in a liquid, so in the case of the home pregnancy test, the marker is hCG and the liquid is urine.

The home pregnancy test is called a Sandwich ELISA because there are three separate zones containing capture antibodies that all perform different things. Scientists decided that a good analogy for this was the humble sandwich, brilliant right?

The first section, or ‘slice of bread’ is called the reaction zone, the second part – the ‘sandwich filling’ – is called the test zone and the last section, the ‘last slice of bread’, is called the control zone. Each of these zones is coated with capture antibodies but they all differ from how they work and what antibodies are placed in the sections.

The Reaction zone

The capture antibodies on the reaction zone, the first slice of bread, will bind only to hCG and will detach from the strip they were on upon exposure to urine. The control zone contains a general antibody, plus a dye and serves to let the user know that the tests outcome.

The urine flows along the absorbent strip, and reaches the reaction zone and it’s capture antibodies. If the urine used has hCG in it, the capture antibodies from the reaction zone will be bound to the hCG molecules found in the urine along with the dye molecules.

The Test zone

When the urine with the hCG and reaction zone antibodies reaches the test zone, there are obviously two possible outcomes. This is the main definitive section of the pregnancy test and determines if you are pregnant or not.  If you are pregnant, the hCG and reaction zone antibody complexes will bind to the test zone antibodies and cause  the dye to change colour (that infamous blue line) If you are not pregnant, then the reaction zone antibodies will pass through without triggering the dye.

The Control zone

The control zone, the last slice of bread, is lined with general capture antibodies and the test culminates here. The general capture antibodies will recognise the hCG and binds to the reaction zone antibodies and releases dye while doing so, creating a second blue line called the Control Line and demonstrating valid test.


When broken down, the pregnancy test is essentially a stick that you pee on, but from a family planning perspective, it can change your life in minutes. It is simple amazing to think of what goes on in that handheld chemistry laboratory, while you are waiting with baited breath to find out the results of something that will undoubtedly change your whole life!