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\title{Middle cerebral artery Doppler improves risk stratification of SGA
babies at a peri-viable gestation}



\author[1]{Lawrence Impey}%
\affil[1]{Oxford University Hospitals NHS Trust}%


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  \date{May 22, 2024}


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DEATH AND SEVERE MORBIDITY IN ISOLATED PERIVIABLE
SMALL-FOR-GESTATIONAL-AGE FETUSES

By Meler et al

Descriptive title:

Middle cerebral artery Doppler improves risk stratification of SGA
babies at a peri-viable gestation

Mini-commentary by Lawrence Impey

Small for gestational age (SGA) babies identified before 26 weeks are a
heterogenous group but the largest contributor is `isolated' SGA'. Most
are `constitutionally' small, but placental issues are common.
Traditionally, the ultrasound Doppler parameters used to identify the
most at risk are the umbilical artery (UA) and uterine artery (UtA).
This paper (Meler et al, BJOG, 2022) challenges the dogma that MCA
Doppler in early onset-SGA babies is of limited use, reporting an 87\%
detection rate for a 14\% false positive rate for UA and MCA together in
predicting a severe composite adverse outcome (CAO).

The analysis uses Doppler findings at referral, thereby reducing but not
eliminating the `intervention paradox', common to many analyses, whereby
an `abnormal' finding's association with an outcome is altered because
it leads to intervention.

The group is defined by local centiles and only comprises those referred
but, by including both apparently FGR and SGA babies, is less subject to
selection bias. Because of the high risk nature and size of this cohort,
the frequency of adverse outcomes is adequate for analysis of a severe
CAO (20.4\%), of death (15.4\%) or long term morbidity that is
sufficiently serious and includes postnatal follow up (minimum 9
months).

The role of MCA Doppler with placental failure is poorly understood.
Near term, as part of the cerebroplacental ratio (CPR), it helps
identify the at-risk SGA baby (Veglia et al, UOG, 2018), and even some
at-risk normally grown babies. Earlier, however, the role of UA Doppler
is clear (Alfirevic et al, Cochrane, 2017). That MCA Doppler adds
predictive value at diagnosis is important because it will allow enable
more appropriate counselling, follow up and potentially better timing of
iatrogenic birth.

What does the analysis make of UtA Doppler and the ductus venosus (DV)?
It is surprising (Allen et al, UOG 2016) that the former was not
predictive, but as its role is well established, this could be the
subject of intervention bias. Mild abnormalities
(PI\textgreater{}95\textsuperscript{th} c) of the DV were not useful,
but severe ones, occurring late in the deterioration in FGR, will still
be useful to time iatrogenic birth (Lees et al, Lancet, 2015).

MCA Doppler in referred small peri-viable babies improves risk
stratification, a process central to maternity care. The `checklist'
approach to risk must be replaced by models using continuous variables
(as opposed to cut offs of `abnormal') of multiple independent risk
factors: as with aneuploidy screening. Only then can we better identify
high risk (sensitivity) whilst not over-medicalising pregnancy
(specificity). Developing this is complex, not least because of the
rarity and gestation-dependence of serious perinatal events and because
of the presence of the intervention paradox in large datasets.
Nevertheless, the Tommy's app (https://www.tommys.org/) is a welcome
start. Such screening is likely to need to be staged, and this analysis
demonstrates one risk factor potentially worth including following a 20
week scan.

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