in IMR, reductions among less developed countries are, on average, much less than those among the more developed countries. Among many low- and middle-income countries, there is also substantial variation in infant mortality rate at a subnational level. A factor of about 67 separate countries with the highest and lowest reported infant mortality rates. The top and bottom five countries by this measure (taken from The World Factbook's 2013 estimates) are shown below. United States Of the 27 most developed countries, the U.S. has the highest Infant Mortality Rate, despite spending much more on health care per capita. Significant racial and socio-economic differences in the United States affect the IMR, in contrast with other developed countries, which have more homogeneous populations. In particular, IMR varies greatly by race in the US. The average IMR for the whole country is therefore not a fair representation of the wide variations that exist between segments of the population. Many theories have been explored as to why these racial differences exist with socio economic factors usually coming out as a reasonable explanation. However, more studies have been conducted around this matter, and the largest advancement is around the idea of stress and how it affects pregnancy. In the 1850s, the infant mortality rate in the United States was estimated at 216.8 per 1,000 babies born for whites and 340.0 per 1,000 for African Americans, but rates have significantly declined in the West in modern times. This declining rate has been mainly due to modern improvements in basic health care, technology, and medical advances. In the last century, the infant mortality rate has decreased by 93%. Overall, the rates have decreased drastically from 20 deaths in 1970 to 6.9 deaths in 2003 (per every 1000 live births). In 2003, the leading causes of infant mortality in the United States were congenital anomalies, disorders related to immaturity, AIDS, and maternal complications. Babies born with low birth weight increased to 8.1% while cigarette smoking during pregnancy declined to 10.2%. This reflected the amount of low birth weights concluding that 12.4% of births from smokers were low birth weights compared with 7.7% of such births from non-smokers. According to the New York Times, "the main reason for the high rate is preterm delivery, and there was a 10% increase in such births from 2000 to 2006." Between 2007 and 2011, however, the preterm birth rate has decreased every