If you've ever run a hand over your baby's head and found sunken bits that feel soft against parts of the skull, you are not alone. These are called fontanelles, and there are several, but the two most defined are at the front of the top of the head (the anterior fontanelle) and at the back of the top of the head (posterior fontanelle).
In newborns, the skull is composed of fairly soft bone plates that can squish together and overlap to get the head through the birth canal during labor. They are flexible for the skull to expand for the developing brain. Given time, these skull plates will fuse, eventually forming the hard protective shell that we know as the skull in adults.
Before then, fontanelles are little gaps between the skull plates. They are soft because they are made up of membranes rather than bone. They are entirely normal, and can be helpful on occasion in our medical assessment.
As the child becomes older, the bones in your child's skull will develop and begin to fuse together, the soft spots will disappear, and the gaps will close. The first one at the back of the head closes by 3 months, and the top one by 18 months.
A very sunken fontanelle - one that appears dipped or shallow - can be a sign of dehydration. If your baby is not eating or drinking very much, is vomiting, has diarrhea, or has fewer wet diapers than usual, you should speak to a doctor urgently.
On the other hand, a bulging fontanelle is also concerning as it can sometimes indicate pressure build-up in the brain or membranes lining the brain. This can be due to head trauma, infection, or build-up of fluid, especially when accompanied by drowsiness, lack of responsiveness, or a fever. This needs immediate medical attention.
It takes skill and experience to determine if a fontanelle is normal or abnormal, so don't worry too much about this. There are other signs that can alert you to how well or hydrated your baby is.
If your child is well and the fontanelle gets bigger or pulses out when they are stressed or crying but returns to normal afterward, this is entirely normal and nothing to worry about.
If the top fontanelle isn't getting smaller by their first birthday, then you should contact your baby's doctor. This could be due to thyroid deficiency, and so may require further investigation.
Despite how soft they feel, the fontanelles are actually stronger than they appear, which means it's ok for you to touch them gently or wash their hair without any consequences.
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