This article evaluates homelessness during pregnancy as a unique, time-dependent risk factor for adverse birth outcomes. Prenatal homelessness was associated with higher adjusted odds of low birth weight and preterm delivery, and lower adjusted mean birth weight. Postnatal homelessness was not associated with these outcomes. Prenatal homelessness is an independent risk factor for low birth weight, rather than merely a marker of adverse maternal and social characteristics associated with homelessness. Targeted interventions to provide housing and health care to homeless women during pregnancy may result in improved birth outcomes.