What Causes A Small Dent On The Forehead After A Fall?

My wife fell why carrying baby, there is some swelling and a small dent on the left top side of her skull, she is acting normal otherwise, we took her to the emergency room and they said they could give her a CT scan or we could wait 24 hr. Since she seem to be acting normally we decided to wait, but I am still concerned about the small dent, what is it , should I be worried?