More women with breast cancer — and an increasing number without — are choosing to have mastectomies over more breast-sparing procedures. And nearly half don’t spend a single night in the hospital but go home the same day, new government data show.

The data, released Monday, add to other studies that show treatment for breast cancer is shifting after years in which women, particularly those with early-stage breast cancer, were encouraged to have less invasive surgeries — and advocates opposed outpatient or “drive-through” mastectomies.

The findings concerned some advocates, who say for many women, the less invasive options are just as effective. The advocates also fear financial issues might be leading some patients to be sent home too soon.

Using data from 13 states that represent a quarter of the population, the federal Agency for Healthcare Research and Quality found that the rate of mastectomy increased by 36 percent from 2005 to 2013. During that same period, the overall incidence of breast cancer remained the same, the report says.

“Even with some of the other papers that are out there, I was surprised by the rapid growth [found in the latest report],” said Claudia Steiner, a senior research physician at AHRQ and lead author of the statistical brief.

Another big change seen in the data: 45 percent of mastectomies in 2013 occurred in hospital-affiliated outpatient surgery centers with no overnight stay. That’s up from 22 percent a decade earlier.

There isn’t data in the report to determine whether women who have their surgeries in a hospital — with a median stay of just over two days — fare better or worse than those who had outpatient surgery. Nor did it speculate on the factors behind the increase in outpatient surgeries, which others say could include patient choice or insurance reimbursement issues and the increasing shift of all types of surgeries to ambulatory settings. All the outpatient centers in the AHRQ study were affiliated with hospitals.

“We know that more procedures in general are moving out of the hospital setting,” said Lisa McGiffert, director of the Safe Patient Project at Consumers Union. “But the reality is we don’t know if outpatient clinics are safer than the hospital because we don’t have much information on quality in the outpatient setting,” added McGiffert, who had not yet seen the study results.

She noted that some outpatient surgery clinics are highly specialized and do the same procedures over and over, and that can lead to improved quality.

About a decade ago, concern about shorter hospital stays following mastectomies prompted about 20 states to pass legislation barring insurers from forcing women to leave the hospital less than 48 hours after surgery. Similar national legislation was introduced a number of times but failed as recently as 2013.

Karuna Jaggar, executive director of Breast Cancer Action, was surprised by the finding, saying she had not heard from her members nationally about outpatient surgeries being on the increase.

If the shift is occurring, “I’m alarmed and concerned,” said Jaggar. “As a patient watchdog group, we would want to know that women are not facing undue pressure,” including the pressure of cost, “to go home before they are ready.”

U.S. Rep. Rosa DeLauro, D-Conn., said Friday she plans to reintroduce a version of the 2013 legislation that would bar insurers from requiring less than a 48-hour hospital stay next month.

“Mastectomies are a major operation and after going through such a physically and emotionally traumatic experience, women should have the ability to continue to stay in the hospital and receive the care they need,” she said in an emailed statement.

The AHRQ research found that the increasing rate of mastectomies was driven mainly by women having double mastectomies, where both breasts are removed. The rate of double mastectomies more than tripled, from 9 per 100,000 women in 2005 to 30 per 100,000 in 2013.

Many of those women had cancer in only one breast but chose to have the other one removed as well, with younger women more likely to do so, the data found.

While most of those having a double mastectomy had a cancer diagnosis, the study found the rate of bilateral mastectomies performed as a preventive measure without any diagnosis of breast cancer more than doubled, although the actual number of women remained small. In 2005, about 2 women per 100,000 without a diagnosis chose the surgery, while that rate rose to 4.4 women per 100,000 in 2013.

But why this rapid increase?

According to other research, that trend is being driven by a host of factors. Some women choose double mastectomies after learning they have a genetic mutation that greatly increases their lifetime risk of breast cancer. Such prophylactic mastectomies garnered widespread media attention in 2013, when actress Angelina Jolie said she had the procedure after learning she carried the BRCA gene mutation, which vastly increases cancer risk.

The National Cancer Institute, the federal government’s cancer research agency, says women with the genetic mutations, as well as those with a strong family history of breast cancer, could consider having a preventive mastectomy. But it also said that many cancer surgeons consider prophylactic surgeries as overly aggressive for women without those risk factors.

“For women with a known gene mutation, that is one situation in which a bilateral mastectomy has been linked to improved outcome and improved overall survival,” said Jaggar. “What’s nonintuitive about this is that for women who do not have a BRCA mutation — and this is far and away most women — there is no data to show that a bilateral mastectomy improves outcome or survival.”

For women with a cancer diagnosis, the choice for a double mastectomy may also be driven by a desire for a more symmetrical look after reconstruction or by fear of recurrence, although statistics show a low likelihood of recurrence of cancer in the second breast.

“Remember, especially for young women, the odds said they weren’t supposed to get cancer in the first place, so they don’t trust statistics,” said Ann Partridge, a medical oncologist and director of the adult survivorship program at the Dana Farber Cancer Institute. She said patients often say they want to do everything they can, even though if a cancer is going to spread, it will likely spread to another part of the body, not the other breast.

Studies have shown that more conservative surgeries — coupled with radiation treatment — are just as effective as mastectomy.

A 2014 study in JAMA, for instance, found that women with early-stage breast cancer who opted for a procedure called a lumpectomy, which is often coupled with radiation treatment for several weeks after, had an 83.2 percent survival rate after 10 years, compared with 81.2 percent for those who had a double mastectomy.

Other papers have also noted the trend toward mastectomies, particularly among younger women. Unlike some earlier papers, Monday’s report includes women with both early-stage and later cancers, as well as women with no diagnosis of cancer at all. The 13 states selected represented 25 percent of the nation’s population, according to Steiner. They included Connecticut, Indiana, Kansas, Maryland, Minnesota, Nebraska, New Jersey, New York, Ohio, South Carolina, Tennessee, Vermont and Wisconsin.