In sub-Saharan Africa, the proportion is about one sixth (some of which can be put down to malaria). These variations alone go some of the way towards explaining the different rates of child malnutrition in the two regions.
But why should low birth weight be so much more common in South Asia?
Low birth weight indicates that the infant was malnourished in the womb and/or that the mother was malnourished during her own infancy, childhood, adolescence, and pregnancy. The proportion of babies born with low birth weight therefore reflects the condition of women, and particularly their health and nutrition, not only during pregnancy but over the whole of their childhood and young lives.
During the pregnancy itself, the average woman should gain about 10 kilos in weight. What evidence there is suggests that most women in Africa probably come close to that figure, whereas most women in South Asia probably gain little more than 5 kilos.
And it is when we close in on this subject that we find the first really significant clues to the South Asian enigma.
Approximately one third of all babies in India are born with low birth weight. In Bangladesh, the proportion is one half.