Body Mass Index (BMI). Body Mass Index (BMI) is measure of body fat based on height and weight that applies to both adult men and women. BMI does not differentiate between body fat and muscle mass. Therefore, body builders and people who have a lot of muscle bulk will have a high BMI but are not overweight or obese.
The bump: or hematoma as we call it, can last for 1-2 weeks. Sometimes it gets slightly worse before it gets better. If the child is acting normally, and not vomiting uncontrollably, she should be fine. If she starts doing those things, call your pediatrician.
Whether or not you decide to use a helmet should depend largely on your child's age and the severity of the problem. For babies under 6 months, the condition will often correct itself with 30 minutes of tummy time each day for a few months; this keeps your baby out of the position that's causing the misshapenness.