By JULIE CRESWELL and REED ABELSON

Hospitals across the nation are being swept up in the biggest wave of mergers since the 1990s, a development that is creating giant hospital systems that could one day dominate American health care and drive up costs.

The consolidations are being driven by a confluence of powerful forces, not least of which is President Obama’s signature health care law, the Affordable Care Act. That law, many experts say, is transforming the economics of health care and pushing a growing number of hospitals into the arms of suitors.

The changes are unfolding with remarkable speed. Two big for-profit hospital chains, Community Health Systems of Tennessee and Health Management Associates of Florida, are combining in a $7.6 billion deal.

In New York City, Mount Sinai Medical Center, which is one of the country’s oldest and largest private nonprofit hospitals, is buying the parent of Beth Israel Medical Center and St. Luke’s and Roosevelt Hospitals. Tenet Healthcare of Dallas, which operates in 10 states, is buying Vanguard Health Systems of Nashville, a network of 28 hospitals and facilities that includes Detroit Medical Center.

In fact, Booz & Company, a consulting firm, predicts that 1,000 of the nation’s roughly 5,000 hospitals could seek out mergers in the next five to seven years.

“There’s immense logic for them to become large super-regional systems, even some national systems,” said David W. Johnson, a managing director for BMO Capital Markets, which advises nonprofit health systems. Some chains are merging to increase their size and their negotiating clout with insurers, while others are trying to reduce costs and improve care, he said.

Some economists and health insurance companies worry that the trend could raise health care costs.

“The rhetoric is all about efficiency,” said Karen Ignagni, the chief executive of America’s Health Insurance Plans, a trade group that represents insurers. “The reality is all about higher prices.”

Whatever the outcome, hospitals are merging faster and in greater numbers than they have in years. After holding steady through much of the 2000s, the number of deals doubled to 105 in 2012 from 50 in 2009, according to Irving Levin Associates, a health care research firm. That is still less than half the annual peak during the last merger wave, in the late 1990s, but Booz and others say this is only the beginning.

Hospital executives say they have little choice but to combine given the coming changes in the industry. Many hospitals are struggling with lower payments from the federal government and declining patient admissions. They are also being confronted with fundamental changes in how they are paid under the Affordable Care Act and by private insurers.

Instead of being paid on volume, rewarded for filling beds and performing more tests and procedures, hospitals are becoming responsible for more of the total cost of a patient’s care. As a result, they have an incentive to keep patients healthy — and out of their facilities.

By combining, hospitals can reduce costs in back-office activities like billing and devote more financial resources to investing in expensive electronic medical records systems and physician practices to better follow patients outside the hospital. Under the new state exchanges created by the federal health care law, consumers will be able to tell the difference in hospital prices between markets that have consolidated and those that have not, Ms. Ignagni said.

The plans have similar designs, but a policy offered by the same insurer in, for instance, Northern California, where hospitals have merged, will be more expensive than one offered in Southern California, where the systems are smaller, she said.

Federal regulators are concerned that the growing number of mergers could lead to anticompetitive practices. The Federal Trade Commission has increased its examination of the deals and has blocked a handful of transactions.

Last year, two hospital chains in Illinois, OSF Healthcare System of Illinois and Rockford Health System, abandoned plans to merge after the F.T.C. challenged the deal on the expectation that the combined hospitals would control 64 percent of acute-care inpatient services. That, regulators said, would allow the combined entity to raise rates and “impose a financial burden on local employers and employees,” through higher insurance premiums, co-pays and out-of-pocket expenses.

For the most part, however, the mergers continue unchallenged among for-profit systems as well as nonprofit hospitals.

Roman Catholic-sponsored systems have formed some of the first national nonprofit chains, and some worry that Catholic hospitals’ growth and acquisitions of non-Catholic hospitals could limit patient access to services like birth control.

In May, for example, Trinity Health and Catholic Health East joined forces to create an 82-hospital system in 21 states. Ascension Health, the largest Catholic nonprofit health system, added 35 hospitals to its network of 78 this year when it closed a deal to acquire a hospital system based in Tulsa, Okla. Ascension has teamed with Oak Hill Capital Partners, a private investment firm, to help finance some of its purchases in the future.

“There isn’t an independent hospital out there that is not thinking about this,” said Gary Ahlquist, a senior partner at Booz, the consulting firm, referring to the mergers. “At the top of the list is the question, Who should I merge with?”

The last wave of mergers took place in the mid-1990s as hospitals joined forces to combat the potential threat from managed care and a giant for-profit chain now called HCA Healthcare. As those threats receded, mergers slowed.

“Today, the motivations for mergers are a lot broader,” said Paul Ginsburg, the president of the Center for Studying Health System Change.

“These independents, or smaller hospitals, are very worried about their ability to do well in the future,” he said.

Analysts say there is not a one-size-fits-all model for this merger wave. While some systems are fanning out across the country, others appear to be focusing on a certain state or region.

But as the hospital merger boom continues, hospitals fiercely holding onto their independence may find it more difficult to compete against bigger, leaner organizations, say analysts.

“There are hospitals out there that have been independent for 80 years and they’re saying, ‘We’re going to be independent for the next 100 years,’ ” said Lisa Goldstein, an analyst of nonprofit hospitals at Moody’s Investors Service. “That’s going to be a tall order. As other hospitals consolidate and grow around you, whatever niche you had will vaporize.”