Why is my doctor concerned about my newborn's large head?

My son had his 2-month checkup yesterday. His doctor was concerned about his head circumference, which was 17 inches. He said he checked to see whether he had any fluid buildup. He said everything appeared normal. What would happen if he did have fluid buildup? What would that mean?

Your son does have quite a large head with a circumference of 17 inches at 2 months of age. The medical term is macrocephaly, meaning simply "large head". It is completely normal for some babies, and can have many causes. The most important factor to watch for in young infants with macrocephaly is a rapid increase in head circumference growth, so your doctor has probably already mentioned to you that he will be following this closely.
One condition associated with a rapid increase in head circumference in infancy is called hydrocephalus, or "water on the brain". That is the fluid build-up that your doctor mentioned. Hydrocephalus is caused by a blockage in the flow of the normal fluid that flows in and around the brain. Treatments are available for this condition. If after the follow-up examination your doctor feels that your baby's head is growing too fast or he finds other changes, he may order a special study of the brain called a CT scan or an MRI.