Yesterday at 3 am when i couldn't sleep i thought why our pregnancies is 40w. If 9months x 4 w =36w ....10x4=40 what do you think???

So what I was thinking if a month is an approximation of 4w by 9 months is 36w and I know that a lot of countries consider that a full term at 36w or 37 w... But we go to 40 🤷 have you ever think about this?

PS: I will ask today in the afternoon to my doctor it make me really curious hahahaha 😂