For decades, Boston University employees have had the option at retirement to convert the lump-sum pension benefit they have earned from the educational institution’s defined contribution plan to a monthly lifetime annuity.

That annuity option “gives people a certain level of security to know they will not outlive their retirement savings,” said Nimet Gundogan, Boston University’s executive director of employees.

Other organizations are more recent adopters to the annuity contract approach, but they realize the importance of doing more than only offering a lump-sum benefit to defined contribution plan participants when they retire.

“Participants are guaranteed a stream of income for life. This is a way of providing a secure benefit for life regardless of ups and downs” in the stock market, said Bill Easterbrook, president of Adventist Healthcare Retirement Plans in Roseville, California, which has been offering an annuity conversion option to defined contribution plan participants since 2012.

Other employers say they intend to consider offering employees the annuity conversion option.

“This will be the next big” retirement plan issue over the next five to 10 years, said Martha Spano, a principal with Buck Consultants at Xerox in Los Angeles.

The number of employers now offering an annuity conversion option to retirement plan participants, typically through their defined contribution plan administrators or through firms that specialize in getting quotes from annuity providers, isn’t known.

But experts say the number is bound to increase due to sea changes that have swept over employer retirement plans.

The University of Wisconsin in Madison is one employer considering enabling retiring employees to choose converting their retirement accounts into annuity contracts.

“One of our goals is that employees are adequately prepared for retirement. A small number of employees might need an annuity,’’ said Rose Stephenson, university benefit program analyst. “We want to make sure all the options are there.”

Retirement annuities are growing in popularity because of the widespread move by employers to freeze traditional pension plans, leaving only the 401(k) or 403(b) defined contribution plans as their employees’ retirement savings vehicles.

As recently as the late 1990s, most large employers offered defined benefit plans. For example, in 1998, according to Towers Watson & Co. surveys, 90% of Fortune 100 companies provided defined benefit plans to new salaried employees.

Last year, though, just 30% of the Fortune 100 still offered a defined benefit plan to new employees, with no sign, experts say, that defined benefit plan coverage will stabilize.

“One day, it may be all defined contribution plans,” said Rob Austin, director of retirement research for Aon Hewitt in Charlotte, North Carolina.

With the decline of defined benefit plans, a core feature of those plans — a fixed monthly pension check promised to retirees regardless of how long they live — will no longer be available to a large percentage of employees.

“The 401(k) plan was once a secondary plan. That is changing as defined benefit plans are being phased out,” said Sri Reddy, head of full service investments at Prudential Retirement in Hartford, Connecticut.

Unlike defined benefit plans in which vested workers must be offered a monthly annuity at retirement, defined contribution plans have no such requirement.

“There is that fear that retirees will outlive” their assets, said Brett Wollam, senior vice president with Fidelity Investments in Smithfield, Rhode Island.

“Some retirees are better equipped to invest than others,” added Tim Holmes, a principal with Vanguard Annuity and Insurance Services in Malvern, Pennsylvania.

Defined contribution plan participants always have the option to take their lump sum benefit at retirement and directly purchase annuities from insurers.

But many will lack the initiative to do that, experts say.

Given that, employers are adding an option to their defined contribution plans in which employees can opt through participating insurers to convert a portion or all of their lump sum balance to a monthly annuity after retirement.

“It is a great enhancement to 401(k) plans,” said Caren Bianco, director of investments with PricewaterhouseCoopers L.L.P. in New York.

In other cases, employers or their retirement plan administrators retain so-called platform vendors, which employees can utilize to get quotes on annuities offered by participating insurers.

“Our system collects quotes,” allowing individuals to compare annuity prices by participating insurers, said Kelli Hueler, CEO of Hueler Investment Services Inc. in Minneapolis.

Available annuities vary widely, from fixed-rate to variable-rate and deferred-income annuities. The most basic annuity is one in which individuals would buy — with all or a portion of their lump sum defined contribution balances — an annuity that pays a fixed monthly benefit for the individual’s lifetime.

If the individual selects a survivor option, a percentage — often 50% — of the monthly benefit continues to be paid to the designated beneficiary, typically a surviving spouse.

Another option is one in which the monthly benefit does not begin until the annuitant, or retiree, reaches a certain age. If the individual were to die before receiving benefits, the designated beneficiary would receive a cash lump sum in the amount the annuitant originally paid.

There is no requirement, experts say, that a defined contribution plan participant use his or her full lump sum to purchase a monthly annuity.

By using a portion of the lump sum to buy an annuity, “the individual has guaranteed retirement income” and can make investment decisions with the remaining portion of the lump sum, said Roberta Rafaloff, vice president of institutional income annuities at MetLife Inc. in New York.

“You are not locked into using your entire” account balance, said Ms. Hueler.

Definitive statistics are not available on what percentage of plan participants who have an annuity option available to them actually utilize it.

“The percentage is still small,” said Robyn Credico, defined contribution leader-North America, with Towers Watson in Arlington, Virginia.

But experts say that percentage is certain to increase, with many employers striving to help their employees achieve income security in retirement.

“In the defined contribution plan world, it always has been about what is the value of the account balance. But what also is important is what the account balance can provide as a monthly” annuity benefit, said Philip Suess, a partner at Mercer L.L.C. in Chicago.