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If you have never talked about retirement with your spouse, you are not alone. But it's a critical conversation if you want your retirement dreams to become reality.

Thirty-six percent of couples said they have not even thought about a retirement plan, and 47 percent of couples disagree about how much money they will need in retirement, according to a Fidelity Investments survey of 1,051 couples in 2015.

How to begin

The million-dollar question that undergirds all other retirement questions is: Are your savings on track to support your desired future lifestyle? Working together to determine this, perhaps using a retirement calculator, can help start the conversation.

Thinking about retirement can be tough for some people. They might associate it with getting old, or a relative's gloomy nursing-home experience. If that's true for your spouse, plan your conversations carefully.

Don't bring up retirement when either of you is stressed or tired, said Syble Solomon, financial behavior specialist and creator of Money Habitudes, an online game for figuring out your attitude toward money.

Instead, talk about it on vacation, or over a nice meal.

Mary Ballin, a certified financial planner with Mosaic Financial Partners in Walnut Creek, Calif., recommends "money dates," where couples set aside time to get away from the house to discuss any money topic.

The power of open-ended questions

Don't make demands when you talk with your spouse about retirement. Instead, ask open-ended, nonthreatening questions.

Another idea is to envision what retirement might look like. Ask your spouse, "When you think about retirement, when you wake up in the morning, what time of day do you think you would wake up? What would you see out the window?" Solomon says.

But don't be surprised if you disagree on some points — or even many points.

"You might have two totally different ideas about what you want to do in retirement," said Shelly-Ann Eweka, a director of financial planning at TIAA in Denver. "There's going to be some compromises."

Getting specific

After some initial discussions, here are three questions to ask each other:

1. What will we do in retirement? People often forget to consider their day-to-day lives, said Rick Kahler, founder of Kahler Financial Group in Rapid City, S.D.

"What I find they don't talk about is, 'What are our days going to look like?' " he said.

For example, maybe you want to travel and your spouse prefers to stay home and garden.

Differences around discretionary spending aren't necessarily a problem, said Jill Fopiano, chief executive at O'Brien Wealth Partners. "In some cases, one of the spouses travels a lot and the other doesn't," she said.

A trickier problem is disagreeing over major financial decisions, she said; for example, one spouse wants to downsize to a smaller house and the other doesn't. Resolving those issues takes compromise.

2. When will we retire? Half of the couples surveyed by Fidelity disagreed on when they will retire — a worrisome finding given that your retirement age is important in determining how much you need to save.

The good news? Figuring out at what age you want to retire can lead to deeper conversations.

Another important question: Will you retire at the same time? The answer can have serious implications for how and when you use retirement savings and claim Social Security.

3. What are our top priorities? Your financial situation will determine what you can do in retirement, so it makes sense to prioritize what's most important.

"Start prioritizing," Eweka says, "and then ask: 'What is it going to take for us to afford it?' "

"Realistically," she adds, "if you have five things on your list, you may be able to get three of them done. That's why you prioritize. Or, you may be lucky — you might be able to do all five."

And if you are behind?

The sooner you start saving, the better, because the power of compounding means your investment returns start building on themselves, making your job a lot easier.

One rule of thumb: If you have a 401(k) or similar plan at work, save enough there to get any company match. That's free money you don't want to pass up.

If you don't have a workplace plan, then setting up an IRA or Roth IRA is the next best thing.

If you are saving something now, consider bumping up your savings rate just a little bit every year. Even small increases can have a big effect over time. See how 1 percent savings hikes can spice up your retired life by $1 million.

Andrea Coombes is a writer at NerdWallet; acoombes@nerdwallet.com.