Millennials just getting used to "adulting" may have a new responsibility to worry about: providing financial support to a parent.

Most of the time it's cash flowing in the opposite direction. A little more than a third of adults ages 21 to 45 get financial support from a parent, with help most often received on cellphone service, food, utilities and insurance, according to a recent survey from financial planning group Society for Grownups.

A Pew study released this spring found that for the first time in more than 130 years, adults age 18 to 34 are more likely to be living in their parents' home than with a spouse or partner in their own household.

But some young workers are taking on sandwich generation responsibilities early. It's a trend that's only starting to gather steam, said Catherine Collinson, president of the Transamerica Center for Retirement Studies.

"We see that many, many Americans who are retired now have relatively little in savings, to make their savings last a lifetime," Collinson said. "The natural fallback for a savings shortfall will be family members, and likely, adult children."

"That tidal wave isn't quite here yet," she said.

Already, 19 percent of millennials are a "financial supporter" of a parent, to the tune of an average $18,250 annually, according to a 2015 TD Ameritrade report. In a new Transamerica survey , 14 percent of millennials cited supporting parents as a current financial priority — double the rate of Generation X workers.

That Society of Grownups survey estimated that 17 percent of respondents are providing financial support to their parents, and 14 percent expect to begin doing so within a year.

Worries about that support have popped up in the data, too. In a recent Schwab survey , 1 in 10 millennials said "taking care of aging parents" is a concern that keeps them up at night, while 40 percent of respondents in a Wells Fargo report said caring for children or aging parents has made their own long-range financial planning more challenging.

"Your pie of take-home pay is 100 percent," said Manisha Thakor, a certified financial planner and the director of wealth strategies for women at The BAM Alliance. "Obviously if you're taking a sliver out of that for your parents, something has got to give."

Here's how to plan if you anticipate helping out your parents sooner, or later:

"For every parent and every adult child it's really important to be having these conversations," Collinson said.

It's better to know in advance that a parent may need or expect financial help, than be suddenly called on to help when his or her retirement savings are depleted or a financial emergency strikes, she said. That helps you assess what kind of help — financial and otherwise — that you can provide without jeopardizing your own financial aims like retirement or putting your kids through college.

Frank discussions may also alleviate adult children's concerns. Half of millennials say it's at least somewhat likely they will provide support to retired parents, the Wells Fargo report found, but only a quarter of boomers said they thought it at least somewhat likely they would need their children's help.

If parents' finances are on shaky ground, it may behoove adult children to help them plan around some of the big retirement decisions, said Thakor. In particular, optimizing Social Security claiming strategies can make a big difference, she said.

Life insurance is something to consider once you have people who would be financially harmed by your passing. Usually, that's a spouse or dependent children, but it could also be a parent you're supporting or who would be left holding the bag on co-signed loans , said Thakor.

Providing financial support to a parent often comes at a time of big financial decisions for adult children, like buying a home and starting a family, said Nondini Naqui, president and chief executive of Society of Grownups.

"This is exactly the time they're feeling the squeeze the most," she said.

Consider how to prioritize your different goals in conjunction with your expected timeline to providing parent help, Naqui said. An imminent need may require forgoing discretionary expenses like a planned vacation; expecting to help 10 years down the line could be a good prompt to focus now on boosting retirement savings and paying off student debt.