Baby measuring below 10th percentile

So I had my first proper midwife appointment today (27+3) she measured me and listened to heartbeat etc. And she’s booked me for a scan tomorrow because the baby measured below the 10th percentile?? Which is super small. With my last baby I had a scan for static growth and that turned out to be nothing. So is this likely to be nothing too? Surely they would of picked up he was small at my 20 week scan if he actually was? Plus I gain a pound a week myself. Is it just because she’s not measured me before?