The skull consists of five thin, curved, bony plates that are held together by fibrous material called sutures. These sutures allow a baby's skull to expand with the growing brain. Usually, the area within a baby's skull doubles in the first 6 months of life and doubles again by age 2. Some sutures begin to close at about this time. After age 2, the skull and brain grow at a much slower rate.

The sutures ossify (gradually harden) to join the skull bones together. The spaces where sutures meet are called fontanelles. At birth, they are covered with a membrane (a thin layer of tissue), which are often called "soft spots." See an illustration of the sutures and fontanelles.

Babies born with certain conditions may have irregular fontanelles and sutures. For example, a baby born with congenital hydrocephalus may have wider sutures than normal, and the tissue covering the fontanelles may bulge.