The ultrasounds can't tell the weight either. They are just a guess. It vary's about 15%.
At 35 and 4 my sono said 6.11 she was born the next morning at 11:30 and was 7.1
My baby was 4 lbs at the 31 week ultra sound! I wouldn't worry! That could be give or take a lb
I dont know why i put sound twice sorry! Lol