It's not very accurate. With my first son I was told he was weighing 7-8lbs and he was really almost 10lbs. I chose to kick him out because it was too uncomfortable but that was at 39w.

I just had a measuring scan last week at 31 weeks, baby A was 3lbs 14oz baby B was 4lbs 7oz 😳 BA is 37th percentile BB is in the 75th percentile 😬