Anyone who has watched a movie or TV show about someone finding out they are pregnant will know the basics. You pee on a stick, get a positive or negative result and hey presto you know that you are pregnant, but how do they work?

Shortly after fertilisation a hormone called human chorionic gonadotropin (hCG) is released. The amount of hCG in the body doubles every two days in pregnant women. This means that the nearer a woman is to her first missed period the more hCG she will have in her system.

Pregnancy tests measure the amount of hCG in a woman’s system. If there is none there will be a negative result. If there is only a little there will also be a negative result. The more hCG in the body the more likely that the pregnancy test will identify it’s presence therefore the further away from conception and into early pregnancy the more hCG will be present and the less ambiguous a positive result will be.

Some pregnancy tests are more sensitive to hCG than others and are often advertised as giving an accurate result X number of days before the first missed period.

A positive result is never wrong but a negative result can be wrong if done too early to pick up on significant levels of hCG so should be repeated a week later if you still have not had a period.