it is okay. my baby was 3.2kg at 37 weeks .. was told baby is slightly bigger than average .. when i did my next appointment a week later at 38weeks, it was 3.1kg .. midwife said it is average .. baby weight during ultrasound is just an estimation .. also, if you have any questions or concerns, you should ask your gynae or midwife .. it is best rather then just go along and be okayy with whatever was told to you .. ask along, there is no stupid qns .. they will give best and moat accurate answer.