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Baby measuring small AGAIN!!!??

So I just left my dr.s office and had my glucose test. All went well from what I know. My baby boy measured 2 weeks behind a few months ago, and at our last appt he measured one week behind and now this time he's measuring almost 3 weeks behind. She said he's either a smaller baby or I carry well since I'm tall. 5'9. Anyone experience this ? She scheduled me to go in tmw for a growth scan ultrasound at the hospital. Should I be worried? ? :(