If you look at doi 10.1257/pol.20140224 you can see that the main difference in mortality is not at day one but late(er) in the first year. So likely the main difference is not due to the distinction between miscarriage and infant death. This is also matched by the fact that children born to middle class parents do statistically as well as kids born to European parents. It is children of poor parents that have the inflated mortality.