The Census Bureau's 2014 Survey of Income and Program Participation included several questions rarely asked by the federal government...
Are you a grandparent? Turns out, being a grandparent is the norm among Americans aged 30 or older who have children aged 15 or older: 61 percent of women and 57 percent of men who meet those qualifications are grandparents.
Do you have children with multiple partners? Among all Americans aged 15 or older, 10.1 percent have children with multiple partners. Among parents aged 15 or older with two or more biological children, 20.6 percent have children with multiple partners.
Have you had children with someone other than your spouse/partner? Among married mothers aged 15 or older who do not have children with their spouse, 17.8 percent have children from a prior relationship and 12.8 percent say their spouse has children from a prior relationship. Among married fathers aged 15 or older, the equivalent figures are 16.5 and 12.6 percent. Among cohabiting parents, the percentages are much larger (50 and 32 percent for women, and 47 and 33 percent for men).
Source: Census Bureau, Fertility Research Brief and Multiple Partner Fertility Research Brief