Pregnancy calculation


I downloaded several apps to find out how many weeks/months im pregnant.

Each app says the same thing that i am 3 weeks and 6 days pregnant which is my 4th week but also all says something different about the date of my next week which is 5.

So one app says week 5 is from aug13-19

The other says from aug20-26 and so on...

Dont know which one is accurate