Low birth weight at term is especially common in South Central Asia, where 21 percent of newborns are affected, accounting for about 64 percent of all affected newborns worldwide. Low birth weight at term is also common in Middle and Western Africa, but much less so in Latin America and the Caribbean. It is important to note that in the poorest developing countries, a large proportion of newborns, and not just those below an arbitrary cutoff point, are likely to suffer some degree of prenatal growth retardation. Micronutrient deficiencies play a part but their specific role in IUGR remains an important research question. Gastroenteritis, intestinal parasitosis, and respiratory infections are prevalent in developing countries and may also have an important effect.
Malaria is a major determinant in countries where that disease is endemic.
Cigarette smoking is becoming an increasingly important factor in some parts of the developing world. The immediate causes of IUGR often operate simultaneously with deeply rooted underlying causes related to the status of women in society, access to quality health services, sanitation, household food security, education, and poverty. The increased risk of mortality extends into the postneonatal period and includes sudden infant death syndrome. Where diarrheal diseases and acute respiratory infections are the major causes of mortality, IUGR infants have shown an increased risk of morbidity. This is because most of these infants' immune functions are impaired.