At the start of pregnancy, levels of the hormone hCG (human chorionic gonadotrophin) rise. The amount of hCG doubles approximately every two to three days and peaks at around seven to 12 weeks into the pregnancy. Pregnancy tests work by detecting the presence of hCG in the urine.

These tests are over 99 per cent accurate and can be used at any time of day, from the first day of a missed period. Some tests can be used up to four days before the next period is due. If testing early, customers should use the first urine of the day. The accuracy in detecting pregnancy decreases the earlier the test is done as levels of hCG may be harder to detect.

If the result was negative but was carried out before a period was late, another test can be carried out on day one of the missed period. Some women may have low levels of hCG during the early stages of pregnancy, so if the period still hasn't arrived within seven days of when it is expected, it is wise to carry out a further test. Examples: Clearblue Pregnancy Test Kit, First Response Early Result Pregnancy Test.

Digital pregnancy tests have the advantage of allowing women to see the result clearly. Some also include details of how many weeks ago conception occurred. Example: Clearblue Digital Pregnancy Test with Conception Indicator.

If the result is positive, it is important for the woman to make an appointment with her GP to start her pregnancy care.