They do say that your pg starts on the first day of your last period, in the 'old' days they thought that was when women concieved, and the traditional 40 weeks has stayed with us!

If you have a longer cycle it will tell you that you a further along and due sooner than you really are.

I found a great website that takes your cycle length and LP into consideration. If you google Pregnancy Expected Due Date Calculator and got to teh site that is called - hpathy - blah blah... you enter the First day of last period, your cycle length and LP length, it will tell you the EDD, the date of likely conception, and how many weeks you are!!!