It is recommended to jump into the purchase of a pregnancy test in order to take it only until the week after you have missed your periods. This helps the test to turn out more accurately avoiding any interpretations and falsely alarming you in the first place. However, if you are getting impatient and refuse to wait until you've missed your monthly cycle, you must then at least wait for one to two weeks after you have had sex to take the most accurate pregnancy test. And, if the results are positive, your body would need time to develop the detectable levels of HCG.