The expectant mother was in labor at South Shore Hospital when she requested a common pain medicine, which was administered by an anesthesiologist. Home with a newborn days later, she was surprised when a bill arrived from the doctor’s group for $2,143.44.

Another patient who went to Emerson Hospital’s emergency department for what turned out to be a broken rib also received a surprise bill: $300.91, for the services of the doctor who read the X-ray.

Neither of the patients initially knew the reason behind the hefty additional charges, according to complaints they filed with state regulators. They sought care at hospitals that were fully covered by their insurance plans. What they didn’t know was that the two doctors were not — and had billed the patients directly for their services.

Getting an out-of-the-blue medical bill — such as when a hospital uses doctors who are outside a patient’s insurance network — has become a nationwide phenomenon. It’s one that has forced exasperated patients to fight with medical providers and insurers at a time when they are already paying for a greater share of their health care.

Patients should not have to “contact their health plan and complain,’’ said David Seltz, executive director of the Massachusetts Health Policy Commission, which monitors health care spending in the state. “Through no fault of their own they are being put in this situation.’’

An analysis by the policy commission found that 10,000 Massachusetts patients in just one year may have received surprise bills for so-called out-of-network care, and policy experts believe that figure underestimates the extent of the problem.

States are increasingly passing laws to protect patients from these charges, such as limiting the dollar amount of out-of-network fees. Massachusetts legislators last year considered doing just that, but failed to pass a sweeping health care bill by the end of the session. A key legislator said approving stricter rules around out-of-network billing is a priority this year. A bipartisan group of US senators is also taking up the issue.

Over the past two years, about 115 patients have filed formal complaints about surprise medical bills with Massachusetts Attorney General Maura Healey’s office, including the two patients at South Shore and Emerson hospitals, according to a Globe review of the documents obtained through a public records request.

Some patients received surprise bills for so-called facility fees, assessed when a patient is treated in an ordinary physician’s office or urgent-care center, but then receives an expensive outpatient bill from the hospital that owns the facility. An investigation by Healey’s office into facility fees charged by Partners HealthCare and its hospitals, including Massachusetts General and Brigham and Women’s, led to a settlement in September requiring Partners to better notify patients of the fees.

More than 35 percent of complaints filed with Healey were over out-of-network charges, which can be up to 200 percent higher than what insurers pay in-network doctors. Among the physicians that were outside the patients’ insurance networks were anesthesiologists assisting in colonoscopies and emergency medicine doctors repairing broken bones and treating heart attacks, something that frustrated patients told Healey’s office they had no way of knowing in advance. Radiologists and pathologists also directly billed patients out-of-network charges.

It’s not unusual for a hospital to have practitioners working in their facilities who are not covered by all their agreements with insurers, a technicality that is often not apparent to patients.

In the South Shore and Emerson cases, the hospitals said out-of-network bills are unusual; most doctors accept the same insurance plans as the hospitals. The physicians’ groups that employ the anesthesiologists and emergency medicine doctors, South Shore Anesthesia Associates and Emerson Emergency Physicians, each said it works with patients and insurers to resolve uncovered charges.

But the Emerson and South Shore patients told Healey’s office that the physicians sent their bills to collection agencies after they refused to pay them. The hospitals and doctors said they could not respond fully to the Globe’s questions about the complaints because the patients’ names were removed by the attorney general’s staff to protect their privacy.

In another complaint, a man who arrived at the Newton-Wellesley Hospital emergency department with a fractured eye socket unknowingly saw an out-of-network doctor who billed him $796.

“Newton-Wellesley Emergency Medicine Specialists works with those patients to decrease the bill to an amount that is comparable to in-network regional payor rates,’’ hospital spokesman John Looney said in a statement.

Another patient with sepsis, a life-threatening infection, went to an undisclosed emergency department, where doctors advised admitting the person. The hospital did not have an open bed, so staff called Northeast Regional Ambulance to take the patient to another hospital. The ambulance company turned out to be outside the patient’s insurance network, resulting in a $2,079.08 bill.

“This was an emergent situation and there was no other ambulance service available within a 50-mile radius,’’ the patient wrote.

Most patients received surprise bills for hundreds of dollars, but a dozen were on the hook for thousands. The highest out-of-network bill reported was $4,600 for lab tests at a Cambridge Health Alliance facility.

Compounding their exasperation, patients often said that despite repeated calls, hospital and insurance company staff could not — or would not — explain the bills. One person wrote of making 20 phone calls, and many patients said they never received return calls.

Some of these complaints may have been resolved after the patient contacted Healey’s office, which tries to mediate disputes.

The policy commission analysis found that about 10,000 patients in 2014 may have ended up with hefty bills for out-of-network care. But that review covered only two insurers, Blue Cross Blue Shield of Massachusetts and Tufts Health Plan, both of which have broad networks of doctors.

“We have no evidence that this has gotten any better,’’ Seltz said.

The problem is more pronounced for consumers insured by large national companies such as Aetna, Cigna, and UnitedHealthcare, which contract with most hospitals but not all local physicians’ groups, experts said. Most of the complaints reviewed by the Globe were filed by patients with national insurers.

Massachusetts has a consumer protection law that provides some recourse for patients, but health care experts and state legislators believe it should be strengthened. And, it does not apply to self-insured companies, which account for 60 percent of patients with commercial insurance in Massachusetts, and are regulated under federal, not state law.

Michael Caljouw, vice president of government and regulatory affairs for Blue Cross, said the insurer will pay the out-of-network bill when the “member did not have a reasonable choice.’’ But he would not say if that includes, for example, a woman who ends up with an out-of-network anesthesiologist during childbirth.

Seltz called these measures “a woefully inadequate solution to the problem.’’ Even if an insurer pays the bill, it is still paying a higher fee to that out-of-network physician, which raises overall health care costs.

That begs the question of why doctors groups do not join all provider networks in the first place.

Several emergency medicine doctors and anesthesiologists blamed insurance companies for the problem, particularly the national companies, which they said want to keep their networks small as a way to shift costs to consumers.

Emerson in Concord contracts with groups of anesthesiologists, emergency medicine doctors, pathologists, and radiologists to provide those services. Dr. Joseph Bergen, Emerson’s chairman of emergency medicine, said that “some of these bigger companies, they will not pay rates that allow us to take care of their patients. We are happy to be in network,we just need to be paid a fair price,’’ he said.

But Aetna and UnitedHealthcare pointed to doctors’ groups, some of which chose not to join insurance networks simply so they can charge exorbitant fees, the companies said.

“We also believe that hospitals have the responsibility to be clear on the network status of all of the health care professionals that serve their patients,’’ said Aetna spokesman Ethan Slavin in a statement.

National doctors’ organizations, insurers, and policy experts agree that consumers should receive advance notice for non-emergency procedures when doctors are not covered by their insurance plan. And, that generally insurers, not patients, should have to pay a fair out-of-network rate. New York and Connecticut are among the states that have passed laws to this effect.

In Massachusetts, the disagreement has come over exactly how that rate is determined. State Senator James Welch, a West Springfield Democrat, said reaching consensus on the issue is a “major priority.’’

“People having health care procedures are in a very vulnerable state,’’ Welch added. “The last thing they need to worry about is any surprise payment.’’

Liz Kowalczyk can be reached at kowalczyk@globe.com.