How soon can a pregnancy test detect pregnancy?

Usually, it can be indicated by a test around the first day of the missed period. Sometimes up to 4 days prior. I am 6 weeks pregnant now and my pregnancy home test detected it the day of my missed period..and if you go to the doctor the day of your missed period and they do a blood test,they will tell you if you are pregnant.Blood test is really the only certain/definate way.. I took a pregnancy test a week before my period was even due, with my first child and it came back positive.