Anytime they tell u they weight of the baby it's a guess. By the docs feeling Ur belly or ultra sound. My 1st baby they said he would be only 4.5-5 lbs, he was 7.9lbs. My 2nd they said (also w ultra sound) 5.5lbs, she was 6.11lbs... Both babies came early at 37/38 wks.
They won't be able to tell you the weight of your baby (it is always an estimate in scans anyway) but what she will do is measure your belly, they then plot their findings on a chart to make sure that your growning at the rate you should be.
They dont. It is a guess! My little girl was 3 weeks early and nurses were all expecting a 5 pd baby and she was 7.6!