fontanel A soft spot of the skull of a newborn infant where the cartilage has not yet hardened into bone between the skull bones. There are normally two fontanels, both in the midline of the skull. The anterior fontanel is well in front of the posterior fontanel. The posterior fontanel closes first, at latest by the age of 8 weeks in a full-term baby. The anterior fontanel closes at around 18 months of age, but it can close normally as early as 9 months. Also known as fontanelle.