A home pregnancy test measures the presence of the hormone human chorionic gonadotrophin (hCG) in your urine. This hormone is first secreted at the time the fertilised egg implants in the uterus (about six days after fertilisation) by the cells which go on to form the placenta. Levels of the hormone build up rapidly in your body in the first few days following implantation. Usually, home-pregnancy tests should be able to pick up the hCG in your urine by the first day you miss your period. Levels of hCG peak between the 60th and 90th days of pregnancy.
If you have a negative result when you first test, it may be that the levels of hCG have not yet reached a level where they can be detected by a test, even by the day your period is due, so you may want to wait a few days then test again.
Not all pregnancy tests are the same. Some are more sensitive and usually more expensive as a result. The more sensitive tests can detect pregnancy even if you have only a small amount of hCG in your system. Concentrations of hCG are reported in milliInternational Units (mIU) or amounts equal to 1/1000th of an IU per millilitre. A test with a sensitivity of 20 IU/L is more sensitive than one with 50 IU/L. You should be able to find this information on any pregnancy test box.
You can test any time of day, and as early as the first day of your missed period for most tests. More sensitive tests may be able to detect low levels of hCG a few days before your period is due but testing this early may give you an inaccurate result (see below). Avoid drinking too much fluid just before testing as this could dilute the pregnancy hormone in your urine. Over-the-counter medicines, such as paracetamol, should not affect the result, but fertility drugs containing hCG may do so.
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