"Necrosis" is a medical term that is used to describe areas of dead tissue. In the case of fat,therefore, "fat necrosis" means areas of dead or dying fat tissue. This is a fairly common problem, in at least three scenarios.
The first, and most serious, is fat necrosis associated with a bad case of pancreatitis. When the pancreas becomes inflamed, sometimes digestive enzymes leak out and can digest the fat surrounding the pancreas, causes fat necrosis. In this case, the fat necrosis is limited to areas that come into direct contact with the enzymes.
The other two common types of fat necrosis are associated with trauma to fatty tissues. The first is fat necrosis of the breast, which commonly occurs after surgery
on the breast, but can also occur with other forms of trauma. The other is subcutaneous fat necrosis of the newborn, which is associated most often with trauma during delivery. Neither breast fat necrosis nor fat necrosis in the newborn are conditions which spread throughout the body, as they both occur locally at the site of trauma.
If you think you might be dealing with a local form of fat necrosis (or if your infant has this), this is something that you should talk about with your primary care doctor
or your pediatrician