How Do Pregnancy Tests Work?

Whether you are hoping to get pregnant, or hoping not to be pregnant, taking a pregnancy test can be a stressful time. But how do home pregnancy tests really work?

Human chorionic gonadotrophin (hCG)

First, a bit of biological background.

Once an egg has been fertilized in one of the Fallopian tubes, it will move into the uterus. Here, the embryo will embed itself into the thickened lining of the uterus so it can develop a placenta to obtain a blood supply from the mother. When an embryo implants into the uterus, it starts to produce high levels of a hormone called human chorionic gonadotrophin, or hCG for short.

Production of hCG does two things. Firstly, it causes the mother to release another hormone called progesterone, which keeps the uterus lining in place (rather than it being shed through the vagina as in a menstrual period). hCG is also thought to have a role in protecting the embryo – which, because it has 50% ‘foreign’ DNA from the father, may otherwise be attacked by the mother’s immune system.

Some of the hCG that is released by the embryo is excreted in the mother’s urine. Pregnancy tests therefore, are designed to give a positive result when hCG in urine is detected.

Here’s how a home pregnancy test works…

The ‘stick’ used for a home pregnancy test contains a strip of absorbent fibers. When the stick is dipped into a sample of urine, the urine moves along the fibers in one direction – just like if you dipped a paper towel in water at one end, you would notice the water creeping along the towel as it is absorbed.

There are three special areas along the length of the pregnancy test strip:

1. First is the ‘reaction zone’. In the reaction zone, the absorbent fibers have been impregnated with a specific molecule called an antibody that attaches itself to hCG – and only hCG.

2. As the urine continues to move along the strip, it carries the antibodies – now attached to hCG – to a second zone called the ‘test zone’. This area contains a chemical that reacts with the antibodies from the reaction zone, but only if they are attached to hCG. If hCG is present, a colored dye is released, causing a colored line to appear on the test strip. If the woman is not pregnant, the antibodies will not have any hCG attached to them, and the dye will not be released, therefore no line will appear on the test strip.

3. The urine continues to move along the strip to the third and final zone – the ‘control zone’. This area contains a chemical that reacts with the antibodies regardless of whether or not they have hCG attached to them. A dye is released and a line appears on the test strip. If the ‘control’ line appears but the ‘test’ line does not, the test has worked properly but you are probably not pregnant. If both lines appear, you probably are pregnant because this shows that hCG has been detected in the test zone.

Because hCG is only produced after the embryo has implanted, pregnancy tests can give a ‘false negative’ result if you take one too early. In other words, if you are pregnant but only just, there may not yet be enough hCG in your urine to be detected by the test. If you are trying to get pregnant, it is therefore usually recommended to wait at least until the first day of your missed period, rather than taking a test straight after you had unprotected sex.

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