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Preterm infants – Definition, classifications and associated risk factors

Preterm birth is defined as infants who are born before 37 weeks gestation1. Preterm births can be further subdivided into categories based on gestational age1:

  • Extremely preterm (28 weeks)
  • Very preterm (<32 weeks)
  • Moderate to late preterm (32 to <37 weeks) 

Other terminology used in these infants include classifying neonates based on birth weight:

  • Low birth weight (LBW) ≤2500g 
  • Very low birth weight (VLBW) ≤1500g
  • Extremely low birth weight (ELBW) ≤1000g

Birth weight based on gestational age: 

  • AGA: appropriate for gestational age 
  • SGA: small for gestational age 
  • LGA: large for gestational age

Preterm birth is the leading cause of morbidity and mortality in children1, with the risk of mortality being inherently linked to gestational age2. Complications associated with preterm births represent the largest direct cause of neonatal deaths, estimated at approximately 27%3

Preterm birth can be divided into two subtypes: 

  1. Spontaneous preterm birth: spontaneous onset of labour <37 weeks gestation or following prelabour premature rupture of membranes, referred to as PPROM3
  2. Initiated preterm birth: induction of labour or elective caesarean section (C-section) <37 weeks gestation due to either obstetric or foetal distress, or both3.   

Risk factors associated with preterm birth:

There are several maternal risk factors that are associated with preterm birth. Maternal history of preterm birth significantly increases the risk of subsequent preterm births. Any underlying maternal chronic medical conditions such as diabetes, anaemia, thyroid disease, hypertension and asthma will predispose the mother to a preterm birth, increasing the risk of preeclampsia for example. It is therefore crucial to identify the presence of such conditions prior to conception, providing appropriate care throughout pregnancy to control the risk3,4. Other factors include age at pregnancy, multiple pregnancy, lifestyle (e.g. smoking, alcohol, drug misuse), nutritional status (e.g. underweight, overweight, nutrient deficiencies), induction of C-section birth, physical and psychological well-being and infection3,4

Certain foetal conditions which may result in a medically indicated preterm birth include preeclampsia, placental abruption, uterine rupture, foetal distress and foetal growth restrictions. 

Advances in neonatal care and medicine have significantly improved the survival rates of these preterm infants, ensuring that they are given the best possible chance of survival. Each member of the MDT plays a vital role in the growth and development of these infants. Being aware of the relevant risk factors of preterm births and the challenges that these infants are faced with, can result in the enhancement of the overall care of this vulnerable cohort. 

  1.  Preterm. World Health Organisation (2018). Available at
  2.  Sarri G, Davies M, Gholitabar M et al,. BMJ 2015;351:h6283 doi: 10.1136/bmj.h6283 (Published 23 November 2015)
  3.  March of Dimes, PMNCH, Save the children, WHO. Born Too Soon: The Global Action Report on Preterm Birth. eds CP Howson, MV Kinney, JE Lawn. World Health Organization. Geneva, 2012.
  4.  Kvalvik L, Wilcox A, Skjærven R et al., BMJ 2020;369:m1007 | doi: 10.1136/bmj.m1007


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