My son measured about 5 weeks ahead on his head, and weight was about 3-4 weeks ahead the only thing that measured on time were his little thigh bones. His legs are still the only "normal" size thing. His head is in the 100% 1/2cm away from being off the chart. Some babies have big heads some have small heads. It's not automatically something to worry about. As long as the blood in the brain is good and flowing correcting stuff like that you should be fine.