Across Massachusetts, about half of primary care doctors aren’t taking new patients, according to the Massachusetts Medical Society’s 2013 Patient Access to Care Study. The rate for internal medicine specialists, or internists, who often also serve as primary care doctors, is 55 percent. If you’ve found a new doctor and want to schedule a routine visit, be prepared to wait. It takes an average of 39 days for new patients to get an appointment with a family physician and 50 days to see an internist. That’s better than last year, when the average wait was a whopping 45 days, but up from 29 days in 2010.

The wait could get longer. The Association of American Medical Colleges projects that nationwide 13,700 more doctors of all types were needed than were available in 2010, and that the gap will hit 130,600 by 2025, with about half of the shortfall in primary care. Are doctors becoming two-headed calves? No, but they are getting scarcer, for lots of reasons.

For one, we’re all living longer, on average, and we need more care as we age. Baby boomers, the oldest of whom are now in their mid-60s, will swell demand for doctors’ time over the next two decades. Doctors are baby boomers, too, meaning many of them are nearing retirement age; about a third of the nation’s doctors, including specialists, are older than 55. Meanwhile, when the state reformed health care in 2006, it expanded insurance coverage, increasing access to care without boosting the supply of doctors.

Nationwide, there simply aren’t enough residency training programs to go around — more than 1,700 newly minted MDs were frozen out of residency programs, primary care and otherwise, in 2013 alone.