Pediatrician and medical school professor Aaron Carroll discusses the causes of high infant mortality in the United States relative to other developed countries. Some proposed explanations don't appear to be supported by research. For example, it doesn't seem to be a lack of prenatal care, and neonatal mortality -- deaths of infants during the first month after birth -- isn't notably worse in the U.S. than in other developed countries. The problem is for infants between one month and one year of age, particularly in low-income families.
(A number of questions occur to me: Does the problem peak at some point and then decline? Does it extend past the first year of life?)
A number of countries send a trained helper (e.g. a kraamzorg in the Netherlands) to assist new mothers, both to do things for them (child care and some housework and making sure the mother's health is OK) and to teach them how to take care of their baby. This might partly explain the difference. The United Kingdom does not have such routine care, and it would be interesting to see how its outcomes compare with those of the Netherlands, France, etc.
For more, see Dr Carroll's article in The New York Times (link).