Most of the top six players -- UnitedHealth Group Inc. (NYSE: UNH ), CIGNA Corp. (NYSE: CI ), WellPoint Inc. (NYSE: WLP ), Aetna Inc. (NYSE: AET ) and Coventry Health Care Inc. (NYSE: CVH ) -- ended 2012 on a high note with impressive earnings growth in the final quarter. Most of the carriers even raised their 2013 earnings estimates. About the Industry The health and medical insurance industry is an integral part of the U.S. economy. According to the Centers for Medicare and Medicaid Services, U.S. health expenditures account for about 18% of the country's annual GDP. According to the World Health Organization, health care expenditure per person in the United States is the highest in the world. Despite huge sums of money spent on health care, millions of Americans lack health insurance coverage or are underinsured.