Premature Infant & Low Birthweight

Defining Prematurity

Many terms are used to describe prematurity. The following represent the 4 new definitions of ‘term’ deliveries: [ACOG: 2013]
  • Early Term: between 37 weeks 0 days and 38 weeks 6 days
  • Full Term: between 39 weeks 0 days and 40 weeks 6 days
  • Late Term: between 41 weeks 0 days and 41 weeks 6 days
  • Post Term: between 42 weeks 0 days and beyond

Other terms for prematurity that are being used include:

  • Premature (Preterm): an infant born at an estimated gestational age of less than 37 weeks
  • Late Preterm: an infant born between 34 - 36 6/7 weeks gestation. “Late preterm” replaces earlier terminology of “near term.”
  • Post Term: an infant born between 42 weeks 0 days and beyond

Describing Age

Terms to define age include:
  • Chronologic Age: time since birth
  • Adjusted Age or Corrected Age: age corrected for prematurity
  • Gestational Age: estimated time since conception
  • Gestational Age Classifications: [Eds: 2012]
    • Moderate to late preterm: 32 to <37 weeks
    • Very preterm: 28 to <32 weeks
    • Extremely preterm: <28 weeks

Classifying Birth Weight

Classification by birth weight: [TeKolste: 2004]
  • Low Birth Weight (LBW): <2500 grams (5 lbs, 8 oz)
  • Very Low Birth Weight (VLBW): <1500 grams (3 lbs, 5 oz)
  • Extremely Low Birth Weight: <1000 grams (2 lbs, 3 oz)


Related Portal Content

Authors & Reviewers

Initial publication: May 2015; last update/revision: June 2023
Current Authors and Reviewers:
Author: Sarah Winter, MD
Authoring history
2020: update: Mary Ann Nelin, MDR; Sarah Winter, MDR
2015: first version: Jennifer Goldman, MD, MRP, FAAPA; Mary Ann Nelin, MDR; Sarah Winter, MDR
AAuthor; CAContributing Author; SASenior Author; RReviewer

Page Bibliography

ACOG Committee Opinion No 579: Definition of term pregnancy.
Obstet Gynecol. 2013;122(5):1139-40. PubMed abstract / Full Text
For the press release of the definition change, see

Eds CP Howson, MV Kinney, JE Lawn. World Health Organization.
Born too soon: the global action report on preterm birth.
March of Dimes, PMNCH, Save the Children, WHO. Geneva, 2012.

TeKolste T, Bragg J, Wendel S.
Extremely Low Birth Weight NICU Graduate.
2004; Washington State Department of Health, Children with Special Health Care Needs Program;
Supplement to: Low Birth Weight Neonatal Intensive Care Graduate. Specifically addresses post-NICU care of ELBW infants who: 1) experienced the usual complications associated with extreme prematurity and/or extreme low birth weight, and 2) were discharged home in a relatively healthy condition.