Confused ?!?

Ashley

Why does my <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">pregnancy apps</a> say I'm 5 weeks and the other say I'm 4 weeks 6 days ? The first day of my last period was January 16. I'm just confused on why some due date calculators say 5 weeks and others say 4 weeks 6 days ?? Anyone know why ?