Final answer:
The sagittal suture, running across the top of the skull back to front, normally begins to fuse late in life. The anterior fontanel near this suture typically closes between nine to 18 months after birth, with the posterior fontanel closing within the first two or three months.
Step-by-step explanation:
The suture that runs across the top of the skull, back to front, is known as the sagittal suture. Typically, the sagittal suture closes in the process called synostosis, starting late in life, as part of the normal aging process. However, during infancy, the areas of connective tissue between skull bones, known as fontanelles, begin to decrease in width during the first year after birth, converting into narrower sutures. Notably, the posterior fontanel usually closes within the first two or three months after birth, and the anterior fontanel closes by nine to 18 months after birth. Premature closure of the sagittal suture, a condition called craniosynostosis, can lead to a distinctive head shape known as scaphocephaly, which accounts for approximately 50% of craniosynostosis abnormalities, and can alter normal brain and skull growth.