How far along

Can some please explain this? I thought pregnancy is counted from the last day first day of your period. I counted it and it should be 7 weeks even my <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">pregnancy app</a> says 7 weeks but when i went to the doctor i found out i was only 5 weeks did they not count the first two weeks?