It’s called Plagiocephaly in medical terminologyPlagiocephaly is a disorder that affects the skull, making the back or side of a baby’s head appear flattened. It is sometimes called deformational plagiocephaly.

There are no symptoms associated with plagiocephaly other than the flattened appearance of the back of the head – either evenly across the back or more on one side. It does not cause any pressure on the baby’s brain and development will not be affected by it in later life. 

Its importance is entirely cosmetic. Nevertheless, if you are worried about any aspect of a baby’s health and are concerned that it might be connected with their head shape, you should always check with a doctor.

https://www.nhs.uk/conditions/plagiocephaly-brachycephaly/