The latest round comes amid flare-ups between Washington and Ottawa and growing, if cautious, optimism in Mexico that the trade agreement will remain.
Talks are running behind schedule and some officials believe the longer they last, the less likely it is that Trump will dump NAFTA, which he has threatened to do if the overhaul of the accord does not benefit the United States.
Negotiators had wanted to wrap up talks by March to avoid them being politicized by Mexico's July presidential election.
But officials have already raised the possibility that they will run past Mexico's vote, and some say they could continue at a technical level for several months if necessary.
A U.S. official noted "there has never been a hard deadline", and among Mexicans following the process, belief is growing that lobbying efforts by U.S. business leaders and politicians to preserve NAFTA has been gaining traction.
"NAFTA will survive, because the economy wins out in the end. The bosses of the big firms are making the argument for us," said Ernesto Ruffo, a Mexican senator from the border state of Baja California. "That's why we're onto the seventh round."
Still, a source familiar with Canadian government thinking said Ottawa had little idea what would happen at the round.
Canada's chief negotiator Steve Verheul struck a downbeat tone last week, telling a business audience "there are large gaps between what we're trying to achieve and what the U.S. is trying to achieve."
But Canada would stay in the talks as long as it saw a chance of striking a deal, Verheul said.
"Heading into the next round we will bring a lot of ideas to the table," he said. "We will be aiming to try to achieve real progress and we're hoping the U.S. will bring a similar attitude."
