Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an
infant of or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal intensive care unit (NICU). They are also at increased risk for long-term health conditions which require follow-up over time.
Birth weight may be classified as:
High birth weight (macrosomia): greater than
Normal weight (term delivery):
Low birth weight: less than
Very low birth weight: less than
Extremely low birth weight: less than
LBW is either caused by preterm birth (that is, a low gestational age at birth, commonly defined as younger than 37 weeks of gestation) or the infant being small for gestational age (that is, a slow prenatal growth rate), or a combination of both.
In general, risk factors in the mother that may contribute to low birth weight include young ages, multiple pregnancies, previous LBW infants, poor nutrition, heart disease or hypertension, untreated celiac disease, substance use disorder, excessive alcohol use, and insufficient prenatal care. It can also be caused by prelabor rupture of membranes. Environmental risk factors include smoking, lead exposure, and other types of air pollutions.
The mechanism of preterm birth is heterogeneous and poorly understood. It may be tied to one or more of the following processes: premature fetal endocrine activation, intrauterine inflammation, over-distension of the uterus, and endometrial bleeding. A prominent risk factor for preterm birth is prior history of preterm delivery. However, there is no reliable protocol for screening and prevention of preterm birth.
Infants born small for gestational age may be constitutionally small, with no associated pathologic process. Others have intrauterine growth restriction (IUGR) due to any of various pathologic processes. Babies with chromosomal abnormalities or other congenital anomalies may manifest IUGR as part of their syndrome.