When a woman becomes pregnant she produces a hormone called HCG. This hormone is only produced during pregnancy and the tests are designed to pick it up. In some women there is not enough of the hormone present to show it up so it will either give a negative or an indecisive result. Some tests are designed to show up a very early result but they are more expensive and not 100% accurate, They are said to be able to detect a pregnancy four days after a missed period.
If you are not sure about the test you can either wait a week or two longer and take another or get a doctor to take a blood test but after a while and you will notice little changes in your body. First your missed period, possible morning sickness or nausea, increased desire to use the bathroom and in some women tender breasts. All these little signs will tell you without doubt that you are pregnant,