Ugh, those 40 weeks can feel like an eternity, especially when you are vomiting your heart out in the first trimester! But yes, a standard pregnancy is calculated as 40 weeks from your last period date.
I just delivered my little one last month at 39 weeks and 2 days. Doctors count a full pregnancy as 40 weeks, but anything from 37 to 42 weeks is considered normal for delivery.
In my experience, my pregnancy was calculated as exactly 40 weeks by my doctor. I delivered at 39 weeks, but the standard calculation is always 40 weeks from your last period.
A full-term pregnancy is 40 weeks. When I was staying at my maternal home, my mother used to count by months, but the doctor always tracked by these 40 weeks!