Baby's head measuring small

Went and had an ultrasound at 29+5 and got told baby's head was measuring in the 36 percentile. The doctor thinks it's because the tech couldn't get a good measurement of her head this time. But at our 20weeks scan she was measuring perfect. So I go back in 4weeks to have another ultrasound to measure. She's been head down since around 20 weeks. Has anyone had this happen? What was the out come?