Soft spot or gap on baby's head (fontanelle)

All babies are born with at least two fontanelles. The anterior fontanelle is a diamond-shaped dip in the top of the head towards the front, measuring about 4cm to 5cm (1.5in to 2in) across. Most babies also have a smaller triangular dip at the back of the head called the posterior fontanelle.

How they are formed

These gaps are formed where the plates, or flat pieces of skull bone, meet. As the baby's head grows, so the plates grow together and the fontanelles disappear. The posterior fontanelle is usually gone by four months and the anterior fontanelle by about 15 months, although it may still be seen on x-rays until two years of age.
Although there's no bone in these spots, the brain is protected by a very tough, thick membrane. The fontanelle may seem to pulsate (it's best to look for this when your baby is quiet), which is simply the pulsation of blood through the veins with every heartbeat.
The fontanelle may also bulge slightly, which is normal if temporary or your baby is crying. However, if the bulge is persistent, see your doctor as this could be a sign that pressure in the skull is abnormally high.
The fontanelle may also sink down considerably - a sign that your baby is dehydrated.

Possible problems

Occasionally, the fontanelles fail to close at the normal time. This is usually a sign of other problems. These are rare but include:

• Hydrocephalus, or 'water on the brain' - increased pressure inside the skull, usually because of abnormal drainage of cerebrospinal fluid, keeps the fontanelles open and causes the baby's head to increase rapidly in size
• Congenital hypothyroidism - a large fontanelle was traditionally used to test for underactivity of the thyroid gland, but blood tests are now done on all newborns to rule this out
• Other rare syndromes, such as dwarfism, developmental delay and bone abnormalities

These problems are rare and you probably would have noticed other symptoms by now. However, if you're still worried, ask your health visitor or doctor to check your baby's head at your next routine visit.
READMORE:http://www.bbc.co.uk/health