Vanishing twins, spared cohorts, and the difference in birthweight between the frailest White and Black infants in the United States

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.