Using Fragile Families and Child Wellbeing Data (N= 4,871), this paper examines why relationship status matters for prenatal health behaviors. The paper argues that a mother’s potential investments in her child’s health are conditioned by socioeconomic and interpersonal resources, including the quality of her relationship with the child’s father. Mothers in strained relationships may experience more stress, which is associated with poor prenatal health behaviors. Results show that married mothers exhibit the healthiest prenatal behaviors and that relationship characteristics and dynamics measures, including physical abuse and relationship conflict, predict poor prenatal health behaviors above and beyond confounding factors. In addition, these relationship characteristics explain some of the advantage in prenatal health behaviors married mothers have over unmarried mothers.