Growth in infancy is a complex process affected directly or indirectly by numerous interrelated factors. The predominant factors include diet, the nutritional status and health of the mother, and the occurrence of infections. In addition, social factors (family structure and cohesiveness), economic status, cultural practices, and biologic factors—such as the sex of the infant, birth weight, birth order, birth interval, and genetics—may also play a significant role in growth.
Nutritional factors may affect growth in infancy both before and after birth. Maternal and infant nutrition are intimately related. For breast-fed infants, nutrition of the mother and that of her young are interrelated from conception until weaning; therefore, dietary intake of pregnant and lactating women is very important.67 Underweight mothers with poor weight gain in the last trimester of pregnancy bear infants with lower mean birth weights.111 Although there are ethnic differences in low birth weight in the United States, the incidence is relatively low.45 In contrast, in developing countries, low birth weight (≤2,500 g), mainly caused by the high proportion of growth-retarded newborns, is a major public health problem.126,138 Infants who have suffered from intrauterine growth retardation remain smaller, on average, than normal babies throughout infancy and early childhood.30,39,44
In developing countries, many nondietary factors also play an important role in determining growth. Infections, particularly gastroenteritis, are among the most important of these factors.91,124 Case-control studies examining factors that differ between malnourished (weight-for-age less than Harvard third or tenth percentile) and adequately nourished children in the second 6 months of life have found significant associations between the following nondietary factors and growth failure: high birth order (greater than six or seven), lower mean maternal age, low maternal weight, children from families in which one or more siblings had died, greater mean number of children less than 5 years old in the home, birth weight less than 2.4 kg, twinning, infections (measles, whooping cough, severe or repeated diarrhea), death of either parent, or a broken marniage.4,96