The best way to count your pregnancy is actually to get an ultrasound. There are known milestones for how large a baby usually is at different points in the pregnancy. Getting an ultrasound will allow your doctor to more accurately determine around when your pregnancy started, and around how far along you are. Anothe way to find out how long you've been pregnant is to ask yourself when your last period was.
The first day of your last period is a good place to start counting how far along you are, but this really only works for those women who have regular, reliable periods that have a pretty consistent cycle. For those who have conditions that cause irregular periods or those who, for some reason, never had regular periods to begin with, this isn’t an incredibly reliable way to count a pregnancy.