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.