E ReView ARticle duration in weeks, measured from the first day of the last menstrual period and, by convention, gestation is recorded as completed weeks and never rounded up. For example, an infant who is born at 32 weeks and 4 days is defined as being 32 weeks. The definition of the "estimated date of confinement" (EDC), also known as the due date, is 40 weeks added to the first day of the last menstrual period and estimates the day when the infant will be born.5 "Postmenstrual age" (PMA) is the time elapsed between the first day of the last menstrual period and the current day, 5 and PMA also can be calculated as the gestational age plus the time elapsed after birth (chronologic age). PMA is used clinically during the perinatal period beginning after the day of birth. "Postconceptual age" is not synonymous with PMA.5 "Corrected age," also called the adjusted age, describes children up to 3 years of age who were born preterm. "Corrected age" represents the age of the child since the EDC. "Term" birth is classically defined as 37 to 42 weeks. Recently, a National Institute of Child Health and Human Development (NICHD)-led group proposed subgrouping births between 37 and 39 weeks' gestation as "early term."6 "Preterm birth" is defined as any birth prior to 37 weeks' completed gestation or fewer than 259 days since the first day of the mother's last menstrual period. 2,3,5,6 Preterm birth often is subdivided further based on birth gestational age. "Late preterm" infants are those born 34 to less than 37 weeks' gestation, "moderate preterm" is designated as 32 to less than 34 weeks' gestation, "very preterm" is designated 28 to less than 32 weeks' gestation, and "extremely preterm," which is the primary focus of this discussion, is less than 28 weeks' gestational age 2,3,5,6 (Table 1). Other useful definitions include "small for gestational age" (SGA), defined as weight less than 10th percentile at a given fetal gestational age. "Large for gestational age" (LGA) is defined as weight greater than the 90th percentile for duration of gestation. Infants greater than 4500 g at term birth are LGA. 7 The terms SGA and LGA age do not distinguish among the various ideologies of these conditions. Low birth weight infants can be further classified into very low birth weight, which includes infants less than 1500 g, and extremely low birth weight infants (ELBW), which comprise infants less than 1000 g.A "live birth" is defined as the complete expulsion or extraction of the product of human conception regardless of the duration of pregnancy, and after such expulsion, the infant breathes or shows other evidence of life such as a beating heart, pulsation of the umbilical cord, or definitive movement of voluntary muscles. "Live birth" does not designate whether the umbilical cord has been cut or the placenta remains attached. Heartbeats need to be distinguished from transient cardiac contractions. Respirations need to be distinguished from fleeting respiratory efforts or gasps. "Fetal death" is death of the product of human conce...