Your pediatrician may decide to do nothing or may order an ultrasound or other exam based on the size and findings of the area. This will rule out other complications like a skull fracture. It can take two weeks to three months for the this bump to go down, this happens as the blood slowly reabsorbs back into the body.
Fluoria, M, Kreiter, S. The Newborn Examination: Part I. Emergencies and Common Abnormalities Involving the Skin, Head, Neck, Chest, and Respiratory and Cardiovascular Systems. Am Fam Physician. 2002 Jan 1;65(1):61-69.
Cephalahematoma. Family Practice Notebook. Accessed 2/2/11 at http://www.fpnotebook.com/nicu/neuro/Cphlhmtm.htm