Objective: Comparison of birthweight references for diagnosing SGA. To provide denominator data for suspicion and diagnosis of SGA. Design: A retrospective cohort study of 10,616 babies. Setting: A regional obstetric centre. Population: 10,616 consecutive newborns, born in Derriford Hospital, University Hospitals Plymouth NHS Trust (UPHT), whilst using the GROW package,1 compared with using Intergrowth 21st (IG21),2 and British 1990 (UK90) references.3 Methods: Statistical analysis of centile data from GROW, IG21 and UK90 references. Main outcomes: Induction rates, detection of suspected and/or diagnosed SGA. Assessment of goodness of fit to the Plymouth population. Results: GROW and IG21 showed bias. GROW had a systematic bias towards smaller centiles (skewness 0.169). IG21 had a systematic bias towards larger centiles (skewness -0.452). UK90 was best fit to the Plymouth dataset with insignificant bias across centiles (skewness -0.047). Conclusions: GROW and IG21 are not appropriate gold standards for our population for allocation of birthweight centile. The size of the population suggests the conclusions may be extrapolatable to other centres. UK90 does not have everyday accessible tools compared with GROW and IG21. A continual local audit of birthweight would be ideal, enabling accurate local centile allocation. If a national SGA screening programme monitoring units’ ability to detect SGA was introduced, it could not start without validated, unit specific birthweight data. Funding: The statistician’s funding was obtained from UHPT Research and Development generic funding.