For more than five years, Michelle Obama's mother, Marian Robinson, has lived with the first family and helped take care of the Obamas' daughters, Sasha and Malia. By definition, their set-up is unique.

Still, aspects of the Obamas' living reflect a broader shift in U.S. society in which grandparents — particularly grandmothers —  are playing an increasingly important role in helping provide child care for working parents.

A paper published by the Journal of Urban Economics in January found married women with kids under the age of 12 were 4 to 10 percent more likely to work if they lived within 25 miles of their mother or mother-in-law.

One of the study’s co-authors, University of Manitoba economics professor Janice Compton, said that while the findings might seem “obvious,” the research also showed that proximity to one’s mother did not make a difference for single women. A single woman's decision to either work or stay home is made independently of whether she has help from her mother, but it was sometimes a decisive factor for married women. In this latter category, the analysis found, roughly 25 percent of women living within 25 miles of their mothers receive work-related child care from them, and 20 percent of women living the same distance from their mothers-in-law.

“Grandparents are the family National Guard; they get called in when there’s a crisis,” said Andrew Cherlin, a Johns Hopkins University sociology and public policy professor who has studied the subject. When it comes to the Obamas, he added, “this is clearly a unique situation that calls for a unique response.”

In fact, importing a grandparent to help out with small children is not uncommon for female White House staffers. Multiple female White House aides have brought their mothers to Washington, D.C., to provide child care for school-aged children over the course of the Obama administration.

While it remains rare for middle and upper-class families to have a grandparent living with them — just 6 percent, according to a 2011 U.S. Census survey — a Pew Research Center study two years earlier found that 39 percent of grandparents had helped in some capacity with child care.

And when it comes to children under the age of 5, Cherlin noted, U.S. Census data indicate that nearly the identical percentage of them have grandparents as their primary caregivers -- 23.7 percent -- compared to day-care facilities -- 23.5 percent.

In low-income families, living with a grandparent has become “an adaptive mechanism to help get through the recession,” according to Pew Research Center senior researcher Gretchen Livingston. One in 10 children in the United States now lives with a grandparent, according to the group’s findings, and nearly half of the grandparents who serve as primary caregivers have household incomes that fall between one and three times the poverty line.

In other words, grandparents remain a critical resource for working parents when it comes to child care -- whether they're living at the nation's best-known address, or somewhere else.