After mergers and acquisitions activity boomed in 2018 to the tune of $3.53 trillion in total deals, nearly half of which were credited to the United States alone ($1.5 trillion), market watchers could fairly place big expectations on this year's output. Not only was last year an unqualified success, but it came on the heels of a strong 2017 ($1.3 trillion in U.S. M&A), and not long after the biggest year on record (2015's total was nearly $1.8 trillion).

But optimism must still be guarded. A number of adverse market trends and developments could put a damper on M&A activity in 2019 and beyond.

“M&A loves certainty, and there are a lot of macro factors out there that are leading to a lack of confidence and to uncertainty,” David Gibbons, a partner at Hogan Lovells, says.

Brexit Woes

As negotiations over Brexit's ultimate form dragged on with a late-March deadline approaching, there was no doubt about the impact of all the uncertainty on deal-making. In the United Kingdom, total deal value plummeted by at least one-third in 2018, Mark Thierfelder, chair of the corporate and securities and private equity practices at global law firm Dechert, says.