My friend's baby had a small head size on an ultrasound. Many tests and visits to specialists later, her doctor realized that she and her husband both have relatively small heads. Baby was completely fine. She wrote about it in the Post, but I cannot find the article.
Good friend had 3 babies with heart issues in utero. All had corrective surgery shortly after birth and turned out just fine.
Run the traps, get it checked out, but it may be nothing and/or completely treatable.