In the industrialized world, low birth-weight babies are usually premature babies. In the developing world, low birth-weigh babies are usually 'full-term' babies; the reason their birth-weight is low is that they have been malnourished in the womb. And one of the main reasons for that is that the mother herself has been malnourished in her pregnancy.
One of the most important facts about infant deaths in the developing world is that the 10 - 15% of babies who are born with low birth-weights (below 2,500 grams) account for between 30% and 40% of all deaths in the first year of life. In other words, low birth-weight babies are approximately three times more likely to die in infancy. Among those who survive, low birth-weight has also been shown to be associated with longer and more frequent illnesses and with mental and physical impairments.