US District Judge John Bates blocked the proposed merger of Aetna Inc and Humana Inc on antitrust grounds. The challenge to the $37 billion merger was among the last major law enforcement actions taken by Obama Administration antitrust officials, reported The Wall Street Journal. In an opinion filed Monday, Bates said federal regulation would most likely be insufficient in preventing the merged firm from raising prices or reducing benefits. Although the decision can be appealed, the outcome could have substantial ramifications on Americans purchasing Medicare and private Medicare Advantage coverage, according to USA Today. "We are reviewing the opinion now and giving serious consideration to an appeal, after putting forward a compelling case," said TJ Crawford, a spokesman for Aetna. The merger agreement will expire February 15, 2017.