Residents have until December 15 to sign up for health insurance through the Massachusetts or Connecticut exchanges if they want coverage to begin January first.

Healthcare officials say they expect many customers will see increases in premium costs — though not everyone.

In Massachusetts, the ConnectorCare program anticipates an average premium increase of 19 percent among people who don’t get government subsidies, though the exact increase varies widely from plan to plan.

In Connecticut, Jim Wadleigh, CEO of the state’s Access Health CT exchange, said people who get subsidies through the federal Affordable Care Act will see almost no change in premiums, though they may have to accept higher deductibles.

Wadleigh said people who pay entirely on their own are likely to see a $75 a month increase, which he blamed on medical trends.

“Customers are either using their health care more than people were expecting,” Wadleigh said. “Pharmaceutical costs are up significantly more than they were a year ago.”

And if President-elect Donald Trump eliminates “Obamacare” entirely, Wadleigh said all bets are off — and no one knows how costs or plan benefits will change.