Almost a third of U.S. households headed by adults under age 55 have at least one stepparent, according to a recent analysis of survey data by University of Massachusetts Boston Professor Emily Wiemers and others. Similarly, the study found that, looking at couples over age 55 who have adult children, 33 percent have a stepchild.

These step-relationships can stretch both the size and definition of family—researchers included both married and unmarried co-habiting couples in the analysis. For Americans with grown children, counting stepchildren boosts the total number of adult kids by 66 percent, the study found.

The rise in divorce and remarriage is driving this growth in family size.