Infants born before completing 26 weeks of gestation account for less than 1% of live births in the United States but more than 40% of infant deaths. The rate of these “periviable” births among non-Hispanic (NH) Blacks exceeds 4 times that among NH Whites. Among periviable infants, small males die most frequently. The mean birthweight of NH White periviable male singletons persistently exceeds that of their NH Black counterparts. The scientific literature includes no explorations of mechanisms that could explain this disparity in birth weight. We offer, and test, the argument that survivors of the vanishing twin syndrome, a phenomenon in which the slower-growing fetus of a twin pair dies in utero, contribute to the disparity. Among male periviable singleton births from 288 monthly conception cohorts (1/95 through 12/2018), we found an average NH White advantage of 30 grams (759g versus 729g). Consistent with our argument, however, cohorts with relatively few survivors of the vanishing twin syndrome showed no disparity.