Introduction
Fetal growth restriction is associated with stillbirth and other adverse perinatal outcomes 1. Antenatal surveillance of fetal growth is based principally on serial assessment of fundal height, and in high risk pregnancy on serial biometry of fetal size, often defined by estimated fetal weight (EFW) 2. The small for gestational age (SGA) fetus represents a significantly increased stillbirth risk 3 and its identification serves as a prompt for further investigations including various Doppler indices at different stages in pregnancy, which are less effective when the fetus is not SGA 4. At birth, SGA is associated with hypoglycaemia and perinatal morbidity.
The conventional definition for SGA has been the 10thcentile for over 50 years 5. Other limits have since been proposed and in general, lower cut-offs such as the 3rd and 5th centiles have been found to have a stronger association with adverse outcome6,7. A fetus with an EFW <3rd centile is more likely to be growth restricted and should be considered for early delivery8 9. However cases above this limit but still below the 10th centile, when defined by customised centiles to exclude constitutional smallness, are also at increased risk 10,11 .
We wanted to investigate limits for fetal growth surveillance, with stillbirth as outcome in low and high-risk pregnancy. The optimal centile cut-off is a compromise between two competing objectives: first, increasing sensitivity – to identify as many cases with adverse outcomes as possible; and second, increasing specificity – to reduce the number of false positives. The optimal point also depends on the standard used to determine the centile and the population being screened. We used the RCOG-recommended 4 customised standard which adjusts for constitutional variation to determine the individual growth potential 12 and better predicts adverse outcome than population based standards13,14,15,16 while reducing false positives17. We undertook the analysis in low vs high risk populations according to early pregnancy assessment, as defined by RCOG4 and NHSE guidelines 8, and as implemented with the Growth Assessment Protocol (GAP)18 in most maternity units in the UK.