Why does everyone say 39 weeks is last week?

Amy • I'm 28, Married to my best friend for 9 years & we have a 6yr old daughter! Now pregnant with our baby boy💙
I keep noticing people say they are on their last week at 39 weeks...for example I have seen a lot of people say "I'm 37 weeks, only 2 weeks left"! This doesn't make sense...pregnancy is 40 weeks! I am 35 weeks & have 5 weeks left! Has anybody else noticed this?