The United States international ranking in infant mortality fell from 12th in the world in 1960, to 23rd in 1990 to 29th in 2004 and 30th in 2005. After decades of decline, the United States infant mortality rate did not decline significantly from 2000 to 2005.Some have argued that infant mortality rates cannot be compared across countries because of differences in the ways that births are registered (some countries don't count very early or very low weight births as "live" births).
The CDC report acknowledges these differences but concludes
There are some differences among countries in the reporting of very small infants who may die soon after birth. However, it appears unlikely that differences in reporting are the primary explanation for the United States’ relatively low international ranking. In 2005, 22 countries had infant mortality rates of 5.0 or below. One would have to assume that these countries did not report more than one-third of their infant deaths for their infant mortality rates to equal or exceed the U.S. rate. This level of underreporting appears unlikely for most developed countries.Even so, the detailed analyses in the CDC report focus on European countries, most of whom use the same reporting standard that the U.S. does, and limit the comparisons to mortality among infants with a gestational age of 22 weeks or more.
The CDC analyses indicate that
The main cause of the United States’ high infant mortality rate when compared with Europe is the very high percentage of preterm births in the United States.However, the report also shows that the U.S. has one of the highest mortality rates for infants born at or near full term (after 36 weeks). For instance, the report compares births between the U.S. and Sweeden. If the U.S. had Sweeden's gestational age distribution, the U.S. infant mortality rate would be about a third lower than it is. However, even this adjusted birth rate would be 30 percent higher than Sweeden's. Put another way, differences in gestational age distributions account for two-thirds of the difference in infant mortality rates between the U.S. and Sweeden, but the remaining one-third is due to higher gestation-specific mortality rates in the U.S.