I'm 15 weeks day and five or six days pregnant, but am I 3 or 4 months pregnant?

We go by weeks. If you are 15 + weeks that means u have 24+ weeks until your due date it is 40 weeks from your last period to your due date, if you go by months it is 10 months from your last period till your due date or 9 1/2 months from the time u became pregnant till your due date.