UnitedHealth Group Inc. (UNH), the biggest U.S. health insurer, raised its 2012 profit forecast as enrollment in its Medicare and Medicaid plans surged in the third quarter and medical costs stayed stable. Earnings may reach $5.20 to $5.25 a share this year, the Minnetonka, Minnesota-based company said in a statement today. That would miss the $5.28 average of seven analyst estimates compiled by Bloomberg. The insurer also said its quarterly earnings rose 23 percent to $1.50 a share, or 5 cents higher than in a preliminary report it gave last week. UnitedHealth added 2.1 million customers to its medical plans from a year earlier, and the share of customer premiums spent on healthcare dropped.