Nonmarital fertility, or having a child outside of marriage, has become an increasingly important phenomenon demographically and socially. The share of births to unmarried women in the United States has nearly quadrupled over the past 40 years: from 11 percent in 1970 to 41 percent in 2010 (Curtin, Ventura, and Martinez, 2014). The increase in births to unmarried women living with a coresidential partner has accounted for much of the increase in nonmarital fertility in recent decades, particularly the first decade of the twenty-first century. Data from 2006–2010 show that over half of all nonmarital births, or one in five of all births, were to cohabiting parents (Curtin, Ventura, and Martinez, 2014). Unmarried parents are on average younger and less advantaged than married parents, their pregnancies are more often unplanned, and their relationships tend to be more fragile (Musick and Michelmore, forthcoming). Recent research on nonmarital fertility has focused on understanding these patterns and their potential implications for family complexity and well-being.