should i be worried?

McKenzie • 20 years young with my hunny garrett. Cant wait for our bundle of joy coming October 3rd (:
so i got my 20 week ultrasound two weeks ago and found out we are having a girl!! yay! although, when they called back about the ultrasound they said everything was normal except the head was a little on the small side..:/ should i be worried! i have been so stressed about it lately. i go in for another ultrasound in 2 weeks!