An extremely low birth weight (ELBW) infant is defined as one with a birth weight of less than 1000g (2lb, 3oz). Most extremely low birth weight infants are also the youngest of premature newborns, usually born at 27 weeks' gestational age or younger. Infants born at less than 1500g are defined as having very low birth weight (VLBW).

Low birth weight (< 2500g) was noted in 8.3% of all births in the United States in 2006, and very low birth weight was noted in 1.48% of all births; approximately 63,137 US births were reported in 2006.

Infants whose weight is appropriate for their gestational ages are termed appropriate for gestational age (AGA). Infants who are heavier than expected are large for gestational age (LGA); conversely, those smaller than expected are considered small for gestational age (SGA) and are also usually found to be intrauterine growth restricted (IUGR) prior to birth.

Extremely low birth weight survival has improved with the widespread use of surfactant agents, maternal steroids, and advancements in neonatal technologies. The minimum age of viability is now as young as 23 weeks' gestation, with scattered reports of survivors born at 21-22 weeks' estimated gestation.

ELBW infants with clinical seizures are at increased risk for adverse neurodevelopmental outcome, independent of multiple confounding factors ¹⁾.