INTRODUCTION
Development of preventive strategies for stillbirth necessitate
recognition that first, the aetiology is heterogeneous and often
unknown, and second, the majority of stillbirths are related to
placental dysfunction, reflected in the coexistence of small for
gestational age (SGA) fetuses and / or preeclampsia. In a prospective
study on screening for adverse obstetric outcomes involving 131,514
women with singleton pregnancies attending for routine pregnancy care at
19-24 weeks’ gestation, there were 477 (0.36%) stillbirths, 92.5% of
which were antepartum and 7.5% intrapartum; placental dysfunction
related stillbirths accounted for 59% of all antepartum
stillbirths1. The dataset was used to develop and
validate a logistic regression model for prediction of placental
dysfunction related stillbirth; a combination of maternal risk factors,
sonographic estimated fetal weight (EFW) and uterine artery pulsatility
index (UtA-PI) predicted, at 10% false positive rate (FPR), 62% of all
cases of placental dysfunction related stillbirths, 70% of those at
<37 weeks’ gestation and 29% of those at ≥37 weeks.
In 93% of the placental dysfunction related stillbirths the birth
weight was below the 10th percentile of The Fetal
Medicine Foundation population charts1,2. It may
therefore be preferable to use one model for prediction of both SGA and
stillbirth, rather than two separate models; the management of
pregnancies at high risk for these conditions is essentially the same
and involves serial ultrasound examinations for early diagnosis of SGA
and then Doppler assessment of the fetal circulation to determine the
best time and mode of delivery. The traditional approach to identify a
high risk group for SGA is the application of a scoring system. For
example, in the UK, according to guidelines by the Royal College of
Obstetricians and Gynaecologists (RCOG), a scoring system is applied to
identify a high risk group for SGA in need of serial ultrasound scans
from 26 weeks onwards3. An alternative method is
provided by our novel two dimensional continuous competing risks model
in which SGA is considered as a spectrum disorder whose severity is
continuously reflected in both the gestational age at delivery (GA) and
Z score in birth weight for gestational age (Z)4-8.
The building block of this model is a patient-specific joint
distribution of Z and GA, that is obtained by combining a history model
with multivariate likelihood of biomarkers according to Bayes
theorem4-8. Risk computation is feasible for any
chosen cut-off in GA and Z, at any stage of pregnancy by adding any
desired biomarker in the same model.
The objective of this study was to examine the predictive performance
for placental dysfunction related stillbirths by the competing risks
model for SGA based on a combination of maternal risk factors, EFW and
UtA-PI and compare the performance to that of our stillbirth-specific
logistic regression model using the same biomarkers 1and to the RCOG guideline for the
investigation and management of the SGA fetus 3.