A pregnancy test will work when pregnancy hormones in the body are already present. This will happen when you have already conceived and implantation has already taken place. A pregnancy test will be testing for the presence of the human chronionic gonadotrophin hormone and it will work only when this hormone is present in the human body. When it comes to timing it will happen after you have conceived, which is about a week after you have had unprotected sexual contact. For the sexually active, it can be challenging to determine when exactly you conceived so it is always best to have it accompanied by other symptoms such as the lack of menstruation, general tiredness, tender breasts and so forth. A pregnancy should take about a week or two to be confirmed and any test done before will bring a false positive or negative, which is not good. In other words, you may have the impression that you are pregnant while you are not or have the impression that you are not pregnant while you are soon going to become a mother. The longer you wait to take this test, the less important it will be to take the test as you will not need it to confirm that you are pregnant.