While some people talk about pregnancy being 9 months long (divided into three trimesters), most health care providers refer to pregnancy as being 40 weeks long. They work this out using the first day of your last period - so that's the day it started. Yes, that is about 14 - 16 days before you even conceived.
You need to figure out when you last got your period. This will count the first day of bleeding as Day 1. Then add 280 days and this will give your Estimated Due Date. In general, all doctors agree that pregnancies, when using the LMP dating technique, average approximately 280 days or 40 weeks.
Only 5 percent of women deliver their babies on their projected due date. 10 percent of first time pregnancies continue on until two weeks past the due date.
So this date should be used as an estimate; it is not an exact calculation.