These tests work by detecting the presence of the pregnancy hormone in your body, called human chorionic gonadotropin (HCG), which is made by the placenta. Women should carry out a pregnancy test on a sample of urine from the first day of a missed period, but some very sensitive pregnancy tests can be used even before this point.
If you don't know when your next period is due, the NHS recommends that you do the test at least 21 days after you last had unprotected sex, and then possibly take another test should your period still not arrive a few days later.
Most pregnancy tests that you buy at the pharmacy or supermarket come in a small box that contains one or two long sticks, which you pee on and then show up the result after a few minutes.
However, all tests are different so make sure that you read the manufacturer's instructions carefully. Used correctly, many tests are more than 99 per cent effective, although if you take the test too early the result can be incorrect.
Therefore, if you take the test and it comes back negative but you still think you could be pregnant, it is worth waiting three days or so and then trying again or speaking to your GP about why you may have missed your period.
Should you take the test and the result is positive, the next step is to speak to your doctor who can advise you on your options at this stage.
Other signs that you may be pregnant are enlarged or tingling breasts, nausea and vomiting, dizziness, a metallic taste in your mouth and a feeling that your period is about to start. However, not all women experience these symptoms so if you've had unprotected sex it's always best to check with a pregnancy test.