Grandma (and/or grandpa) is the single most common primary day care provider for the nation's preschoolers with working mothers.
Among children under age 5 with an employed mother, 21 percent were cared for primarily by a grandparent. In second place was the father (20 percent). Day care centers were third, at 19 percent.
Source: Census Bureau, Who's Minding the Kids, Child Care Arrangements: Spring 2010