Ok I hope I dont confuse you when I say this lol or myself!
Ok so the DR will tell you you are say 38 weeks pregnant but what you dont ever hear them say is although you are 38 weeks along the baby's or babies

gestation age is 36 weeks. So that is taken into considereation I guess thats just the easiest way to do it! The way most DR calculate you due date is by taking the first day of your LMP and subtracting 3 months from it then adding 7 days. (Just in case you are wondering) I always found that those online due date calculators were off from my actual due date. I wish I could be more help lol