<Li> The mastoid fontanelle closes next from 6 to 18 months after birth; and </Li> <Li> The anterior fontanelle is generally the last to close between 18--24 months . </Li> <P> The fontanelle may pulsate, and although the precise cause of this is not known, it is perfectly normal and seems to echo the heartbeat, perhaps via the arterial pulse within the brain vasculature, or in the meninges . This pulsating action is how the soft spot got its name--fontanelle is borrowed from the old French word fontenele, which is a diminutive of fontaine, meaning "spring". It is assumed that the term spring is used because of the analogy of the dent in a rock or earth where a spring arises . </P> <P> Parents may worry that their infant may be more prone to injury at the fontanelles . In fact, although they may colloquially be called "soft - spots", the membrane covering the fontanelles is extremely tough and difficult to penetrate . </P>

When does a baby soft spot fill in