Fontanelle

One of the 2 membranecovered spaces between the bones of a baby’s skull. At birth, the skull bones are not yet fully fused, and 2 soft areas can be felt through the scalp. These are the anterior fontanelle, which is diamond-shaped and usually closes up by age 18 months, and the posterior fontanelle, which is triangular and closes up within the first 2 months. It is normal for the fontanelles to become tense and bulge out when a baby cries. Persistent tension at other times may indicate an abnormality, particularly hydrocephalus (the accumulation of fluid in the skull). A sunken fontanelle may be a sign of dehydration. If a fontanelle is abnormally large, or takes a long time to close, the cause may be a brain abnormality or a disorder, such as rickets, affecting the skull bones. Early closure of the fontanelles results in a deformity called craniosynostosis. Occasionally, a third fontanelle is present between the other 2; this occurs in Down’s syndrome.