"An infant described as 'low birth weight' weighs less than 2500 grams at birth; a 'very low birth weight' infant weighs less than 1500 grams. Low birth weight is a major factor in infant mortality in the U.S. Infants weighing 2500 grams or less are almost 40 times more likely to die during their first 4 weeks of life than the normal birth weight infant. Low birth weight infants are 5 times more likely than normal birth weight infants to die later in the first year and account for 20% of postneonatal deaths. The two major contributors to low birth weight are preterm birth and intrauterine growth retardation (IUGR). Both contribute to inadequate fetal growth. A birth is considered preterm if it has a duration of less than 37 weeks from the last menstrual period. IUGR refers to low weight for a given duration of gestation."
Zuckerman, Barry, "Drug-Exposed Infants: Understanding the Medical Risk," The Future of Children, Spring 1991, Volume 1, Number 1, p. 28. Citing Institute of Medicine, Preventing Low Birthweight: Summary, National Academy Press, Washington, D.C., 1985.