BOSTON — A federal mandate requiring insurers to cover birth control is under threat from President Donald Trump and the Republican-controlled Congress, according to women's advocacy groups who want the state to preserve the benefit.

The Affordable Care Act requires insurance companies to cover all methods of birth control approved by the Food and Drug Administration, with no co-payment or co-insurance when provided in network. The Massachusetts health care law, which went into effect in 2006, doesn’t include such a provision.

Advocates want lawmakers and Gov. Charlie Baker to add the mandate to ensure women continue to receive coverage. While repeal of the federal health care law is off the table for the moment, advocates say the Trump administration is still tinkering with provisions of the law that mandate coverage.

One proposal being discussed in Washington would allow employers to opt out of providing contraceptive coverage for religious or moral reasons.

"Too many women in Massachusetts are teetering on the edge of losing their basic protections because of what's happening in Washington," Rebecca Hart Holder, executive director for NARAL Pro-Choice Massachusetts, said at a briefing Tuesday. "Women's health care should not be used as political football."

The Beacon Hill proposal is backed by more than 70 lawmakers as well as the Massachusetts Association of Health Plans, which represents private insurers.

"Women feel under siege, and we don't know what's going to come out of Washington," said Lora Pellegrini, the association's president and CEO. "But we don't want to erode protections afforded to women under the Affordable Care Act. We want women to know that the health plans are on their side."

Under the state proposal, which was heard by a legislative committee Tuesday, coverage without co-pays would be guaranteed for emergency contraception and for all FDA-approved methods of birth control.

Insurers also would be required to cover a year's supply of birth control after an initial three-month prescription, which is not required by the federal law.

Recent studies show the federal birth control mandate, in place since 2013, has increased use of contraception and reduced out-of-pocket medical costs, particularly for long-acting methods such as intrauterine devices.

From 2011 to 2014, out-of-pocket spending on birth control in Massachusetts dropped 81 percent, according to the state's Health Policy Commission.

Massachusetts has some of the highest health care costs in the country, and state officials have wrestled with ways to reduce them.

A study by the state Center for Health Information and Analysis last year determined that requiring plans to include birth control coverage under state law would increase monthly health premiums by an average of 15 to 30 cents.

But supporters of the mandate say that study is flawed because it neglects the increased cost of unwanted pregnancy, sexually transmitted diseases and other issues that could stem from a loss of contraception coverage.

Until recently, most health plans were reluctant to support the state mandate amid concerns over rising costs. A similar bill introduced in the previous legislative session failed to win support.

Under the latest proposal, insurers could not charge co-payments for generic birth control drugs but would be allowed to charge for brand-name medicine, helping them to contain costs.

Trump hasn’t specifically targeted the federal birth control mandate but Vice President Mike Pence, who had a long anti-abortion record as a a conservative Republican governor from Indiana, has indicated he wants to eliminate the provision. So have conservative Republicans in Congress.

Twenty-eight states — including California, Illinois, Maryland and Vermont — have required contraceptive coverage, according to the National Conference of State Legislatures. Most require insurers to cover all FDA-approved methods; while others require coverage with no cost sharing.

Carol Rose, executive director of the Massachusetts chapter of the American Civil Liberties Union, said women's reproductive rights are under attack.

She urged lawmakers and Baker to "protect women's access to birth control" by swiftly approving the proposed legislation.

"We're facing a presidential administration that wants to strip away rights that people fought so hard for," she said. "This is urgent. All eyes are on Beacon Hill."

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites. Email him at cwade@cnhi.com.

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