A normal pregnancy is 40 weeks long. People say a pregnancy is 9months but it is really 10months if you are counting by the weeks. Your healthcare provider will count your pregnancy by the weeks.
The pregnancy is also divided into trimesters:
First Trimester: 1-13weeks
Second Trimester: 14-27weeks
Third Trimester: 28-40weeks
Your due date is just an educated guess based on your last menstrual period and/or by a dating ultrasound. It is common for your due date to be off by two weeks in either direction. A pregnancy is considered full term in medical standards when you reach 39weeks gestation.