Because my pregnancy was not achieved naturally, I know the EXACT date of my baby's "conception". They tack on an automatic two weeks because that is the standard that was set up. When my zygote/fetus--whatever it was at the time--was actually four weeks old, it registered on the charts as six weeks. The system that doctors use to measure size and development is based on menstrual cycles. Assuming that each woman is most likely to become pregnant at a certain point in her cycle (and because most people don't know the exact date of conception), the doctor counts two weeks from your last period as your date of conception. Then, everything after that is based on what's been recorded for that period of time. When my fetus was actually eight weeks, it registered as 10 weeks and was doing what 10wo fetuses do, because the chart/system has been set up to incorporate an automatic two weeks. On my due date, my fetus would have been actually 38 weeks old, but I would have been 40 weeks pregnant.
Did I clear that up at all? Can you understand why there's a bit of a gray area? During my ultrasound appointments, the techs and "guest" doctors would try to tell me stuff about dates, not knowing that I had exact info because I was in on it from before Day One.