NEW YORK (TheStreet) -- Knowing how much your significant other earns can make a big difference in how much money gets saved for retirement as a family. "When you are clear on how much each spouse earns, it creates a cap on spending and a reality around what you can afford," said John Sweeney, executive vice president of retirement & investing strategies with Fidelity Investments.

Some 72% of couples say they communicate exceptionally or very well about money matters, but 43% could not correctly identify their partner's salary, and 10% were off by $25,000 or more, according to Fidelity Investments' 2015 Couples Retirement Study. "It appears that couples have gaps in communication, and because incomes are shifting from year to year due to the freelance economy, it creates further challenges for couples to predict their own income," Sweeney said.

Another surprising finding of the study was that about a third of couples disagreed on how much they had in investible assets.

"When building a life together, couples need to be in alignment on social, emotional and financial factors," Sweeney said. "If not, it can create stress in the relationship."

To be exact, 36% of couples disagreed on the amount of the household's investible assets, and when asked how much they will need to save to maintain their current lifestyle in retirement, 47% disagreed.

"The missing piece is planning," said Sweeney. "If couples aren't aware of what they are making and spending, they will not know how much they need in retirement."

The level of disagreement was highest in the demographic that is closest to retirement: baby boomers. "Longevity, inflation and health care costs are the three unknowns that can make retirement planning a challenge," said Sweeney. "The rising cost of health care could become a fixed expense like food for some people. It's not an expense you may have budgeted for."

It is especially important for women who are part of a couple to seek financial clarity with their partners, because they tend to live longer. "Women need to have as much or even more interest in their family's financial matters as men," said Sandy Chaikin, co-founder of Chaikin Analytics. The average U.S. man will live to be an average age of 76.2 compared to 81.3 for the average woman, according to a 2012 report from the Institute for Health Metrics and Evaluation. "Since it's a fact that most women will outlive their partners, they need to have worked to save a nest egg and have investments that can support them once they retire and/or are on their own," Chaikin said.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.