"Microsomia" is defined as

A neonate whose weight falls between the 10th and <90th percentile for gestational age is considered as Appropriate for gestational age (AGA). If the weight falls below the 10th percentile, the neonate is classified as Small for gestational age (SGA) or microsomic. If the weight falls at or above the 90th percentile, then the neonate is classified as Large for gestational age (LGA) or macrosomic. Reference : page 137 Ghai Essential Pediatrics 8th edition