Head circumference measuring at 33 weeks

I had my 7th month appointment today and as always my doctor does an ultrasound to check on the baby. Today he told he that his head was measuring at 33 weeks instead of 29 weeks. Have any of you mommies gone through something like this? If so, what was it? My husband thinks I'm overreacting, but it just worries me that there might be something wrong. I really appreciate your comments.