Baby may just be big but you may have ovulated earlier then what the doctors think. A due day is never accurate, its just a guess. If you go online do a search for different types of due date calculator.
Ya if your periods are not always the same # of days then its hard to tell sometimes but its normally pretty accurate to go off the ultrasound as the earlier ones are most accurate. My dr based my due date off when I ovulated since my periods ranged from 26-31 days. If you know when you ovulated then you can base it off that but I would say you are 17 weeks. Again like the pp said they are really just a guess and honestly on 3-4% of babies are due on their due date.
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