Confused by number of weeks!!! Help!!

Joanne
Hello!! I've been counting my weeks religiously and got to 16 weeks today - I decided today to put each week on my calender, and my due date takes me to 42 weeks?!?! Can anyone explain this?? I don't understand!!! I thought it was 40 weeks, I can't be 2 weeks out with all my appointments etc!! Thanks in advance guys!!! My tummy tells me I'm definitely 16 weeks!! 😁😁