Your baby might be suffering from dandruff. Yes, it may sound funny to you but even babies as young as months old are prone to dandruff.
This is called "cradle cap". This kind of rash covers a large portion of your baby's head that it resembles like a cap, hence the name.
Cradle cap may be due to do the overactive sebaceous glands in the skin of newborn babies brought about by the mother's hormones that remain in the baby's circulation. It is itchy and babies tend to scratch the rash which would at times result to irritation.
What you can do is to use a mild shampoo and gently brush your baby's scalp with a soft brush until the flakes fall off.
However, there are cases in which the rash is hard to remove and gentle oil can help like olive oil.
Just rub a small amount of oil to the rash and gently brush the area with a soft brush.
Then, wash your baby's scalp with a gentle baby shampoo to avoid the oil to stick to the baby's head as this will further aggravate the condition.
However, if the condition persists and everything seems futile, do consult your doctor.
I often recommend using anti-seborrhea soaps or wash and a steroid of low potency to be applied daily.
Cradle cap disappears in due time as the baby matures. As long as the rash is well taken cared from the moment it erupts, it will not worsen. Cleanliness and good diet result to good, healthy skin.