Different limits are used in practice to define SGA, including 10th , 5th or 3rd centile, or – more rarely – 1 or 2 standard deviations (SD). The lower the limit, the stronger the association with pathological outcome. However, low limits e.g. <3rd miss other cases (for example between the 3rd and 10th centile) that are also at significantly increased risk 1.
For obstetric and midwifery screening / assessment and prospective management of pregnancy, the 10th centile is recommended to define SGA 2,3
1. Clausson B, Gardosi J, Francis A, Cnattingius S. Perinatal outcome in SGA births defined by customised versus population-based birthweight standards.
Br J Obstet Gynaecol. 2001;108(8):830–834. https://doi.org/10.1111/j.1471-0528.2001.00205.x
2. Gardosi J. Intrauterine growth restriction: new standards for assessing adverse outcome.
Best Practice & Research Clinical Obstetrics and Gynaecology; 2009: 23; 741–749 https://doi.org/10.1016/j.bpobgyn.2009.09.001
3. Royal College of Obstetricians and Gynaecologists The Investigation and management of the Small-for-Gestational Age Fetus:
Green-top Guideline No. 31: 2nd Edition. RCOG 2013 https://www.rcog.org.uk/globalassets/documents/guidelines/gtg_31.pdf