Strengths and limitations
The strengths of our study include the use of a large database derived
from routine, prospective data entry in a national programme, and the
application of a customised standard which adjusts for constitutional
variation and thereby better reflects the fetus’ growth potential25 . In a proportion of cases the early pregnancy risk
assessment was not recorded, as the data field was not contained in the
electronic maternity record. However, the similar stillbirth rate in
this ‘missing’ group suggests that this did not introduce a systematic
bias.
We had no ultrasound measurements recorded in this database and
therefore the trajectory of growth, expressed as velocity or growth
rate, could not be calculated. While serial scans can provide useful
information about growth rate 26, it has also been
argued that it adds little to the effectiveness of the last scan to
predict adverse outcome 27–30. The use of
birthweight-based centiles could be considered a weakness of the study,
as the objective was to predict risk of stillbirth based on fetal rather
than neonatal weight. However, it can also be considered a strength, as
1. it allowed comparison between subgroups (the low risk group having
had no clinical indication to undergo ultrasound investigations); 2.
There was no need to adjust case by case for the respective
scan-to-delivery interval; and 3. it is appropriate to infer fetal
weight from newborn weight as gold standard, as estimated fetal weight
is prone to systematic and random measurement error31. It was possible to apply this method as the
customised GROW chart provides the same, contiguous standard for fetal
and neonatal weight.