Baby head measuring small.

Went went for our 20 week scan a couple of weeks ago and the sonographer mentioned that the head was measuring small about a week behind. She scheduled us for another scan today and the head had grown but is still showing as a week behind and in the 3rd percentile. Im 22 + 5 but it's measuring at 21 +3. Myself and my husband are not particularly large people and I have petite features so we are just hoping it's a small baby. We have another scan booked for 2 weeks. Has anyone had anything similar happen at a scan?