What happens if....

What happens if my baby has a small head? I'm 22 +3 weeks now and yesterday at the Doctors they brought up how my baby's head is measuring small and in 4 weeks I'll be needing another ultrasound to see if it was just how he was moving or if it's grown or if it's still small. She didn't go into details about what would be done if his head is small and I didn't really know what to ask. Have any of you mommas been through this??