Health insurer Cigna Corp. has announced plans to buy pharmacy benefit manager Express Scripts Holding Co. for $52 billion, the latest tie-up in the health care industry.

But in joining forces, the companies may not have an easy road in front of them. That’s because both of the U.S. government’s antitrust agencies are likely to continue their trend of aggressively challenging health care mergers, according to a new Crowell & Moring report on regulatory challenges various industries will face this year.