Ultrasounds can be off and it does sound like your baby has microcephaly, HOWEVER..... babies sometimes are off on growth at the younger age and then catch up quickly.
For example, my uterus measured 6 WEEKS behind at one point, I was 32wpregnant and measured only around 26w. Then about 4 weeks later I had caught up completely.
Baby can catch up, it's DEFINITELY possible, also sometimes kids just have small heads and nothing is really wrong. :)
have faith, talk to your doctor :)
I think the only person that can answer this question is your doctor... Ultrasounds can be off but it could definitely be an indication that there is something wrong with the baby. The only way you are going to know for sure is to have the baby and see what happens afterwords. I hope that everything turns out okay for you and baby.