As the Pregnancy Discrimination Act of 1978 (PDA) turns forty, it is time to consider how we define pregnancy discrimination. In recent years, courts have come to define pregnancy discrimination almost exclusively through comparison. Yet our understanding of discrimination, inside and outside the pregnancy context, depends on judgments about social roles as well as comparison. Both Congress and the Court appealed to social roles in defining the wrongs of pregnancy discrimination. In enacting the PDA, Congress repudiated employment practices premised on the view that motherhood is the end of women’s labor force participation, and affirmed a world in which women as well as men would combine work and family—a world in which pregnancy would be ...