Dads are a more popular option, too, over day care centers and nursery schools, as an increasing number of mothers resume working and many families struggle to pay rising child care costs.
"Economics play a huge part, but as well in today's society, grandparents can provide a stability and root that are terribly important," said Sue Johnson, of Lancaster, Va., co-author of a book with her daughter-in-law on raising grandchildren.
Donna Butts, executive director of the advocacy group Generations United, said children benefit when cared for by a relative.
"It all has to do with the connection with the child, that familial connection for parents who feel more comfortable with relative care," Butts said.
In 1997, a grandparent had primary care of 18 percent of children under age 5 while the youngsters' mothers worked, according to the latest available data from the Census Bureau. That compares with 16 percent in 1985, the earliest year for which similar data was available.
By comparison, day care centers watched 17 percent of children in 1997, up from 14 percent in 1985, though percentages for both groups fluctuated in the survey years in between. Meanwhile, nursery schools, preschools and home day care arrangements all became less popular options over the full 12-year span.
"I feel very lucky. Most of my friends don't have the opportunity that I have," said Frances McClennen, who once a week drives two hours from her home on Cape Cod, Mass., to a Boston suburb to watch her two daughters' four young children.
The proportion of children being watched by fathers while the mothers worked also increased, from 16 percent to 19 percent.
"These aren't necessarily all stay-at-home dads. It might be fathers adjusting their schedule while the mother works a part-time shift, or the mother working a day shift while the father works off-hours," said Census Bureau analyst Kristin Smith.
Looking at all children under 5, regardless of the mother's working status, 21 percent were regularly cared for by a grandparent, and 17 percent by the father. An additional 13 percent were cared for by a nonrelative in the provider's home, and 12 percent were in day care centers.
Costs play a role, too, the census report shows. For instance, working mothers on average paid a day care facility $3.52 an hour to watch their kids, while most grandparents do it for free. As a result, mothers living below poverty level tended to rely more on grandparents, the father, or another relative to take care of the child.
"We know that really high-quality day care centers are really good for kids. However, there's not a lot of great high-quality centers out there, and they are very expensive," said Claire Lerner, child development specialist for ZERO TO THREE, a group devoted to the healthy development of babies and toddlers.
"So it's only natural that more parents look to grandparents for help," she said.
Among minority groups, Asians and Pacific Islander households, which usually include more extended families in the house, had higher rates of grandparents helping with raising kids.