Fetal MRI Estimates Fetal Weight Centiles Closer To True Birthweight Than Ultrasound

1. Background

Correctly estimating fetal size= Clinically important

Antenatal fetal size = Estimated Fetal Weight (EFW)

Current gold standard is Ultrasound Scan (US)

Both Small and Large for Gestational Age (SGA, LGA,) fetus

→ Birth complications

As per Big Baby Trial, diagnosis of LGA by US has large variation

& false positives 60% (Gardosi et al., 2025)

Scope for improving diagnostic accuracy, particularly in the LGA

cohort.

There is emerging evidence that MRI may be more accurate

at predicting fetal weight compared to US.

 

2.1. Methods

As part of the currently recruiting MibirthResearch Study

(www.mibirthstudy.com) 131 singleton participants,

between 35+5 and 40+6 weeks gestation,

had the following within 48 hours of each other:

1. Antenatal fetal growth USS

2. Low field MRI (0.55Tesla)

Estimated fetal weight was calculated using both modalities (see 2.2.)

All US, MRI and final birth weights plotted on Intergrowth 21st charts

to obtain centiles and then compared.

5. Summary

Both US and MRI tended to provide estimates of fetal weight centile at late gestation

higher than the actual birthweight centile. This may represent overestimation, MRI

less so than US.

At gestational ages over and including 35+5, MRI was more likely to provide an

EFW centile closer to the birth centile than US.

Further data (n=500) will be collected in Mibirth Research Study.