There are several factors that contribute to low birth weight. These include but are not limited to poor prenatal care, poor nutrition, smoking, drug use, and underage mothers. I suspect it is those factors that cause some racial groups to have higher infant mortality rates. Recent studies have shown a decrease in teenage pregnancies in most racial groups, but African-American and (if I remember correctly) Hispanic teens are more likely to get pregnant. They are also more likely to have other high risk factors, leading to more problems with infant mortality.

As for the comparisons between nations, such comparisons can only be as accurate as the underlying definitions and statistics. If nations use different criteria to calculate their infant mortality rates, then no valid comparison can be made.