To evaluate the contribution of fetal cerebral Magnetic Resonance Imaging (MRI) in cases of structurally isolated macrocrania detected by prenatal ultrasound and to assess the impact of ultrasound biometric chart selection on MRI referral patterns.

We conducted a retrospective, single-center study (January 2011–June 2022) in a tertiary fetal medicine unit. Isolated macrocrania was defined as an ultrasound head circumference ≥97.5th centile with measurements interpreted using the Intergrowth-21st, CFEF, or WHO charts. Prenatal ultrasound findings, fetal brain MRI results, and pregnancy outcomes were systematically analyzed, and MRI referral rates were stratified by the biometric chart selection.

Among 770 fetuses with ultrasound-identified macrocrania, 576 were structurally isolated cases, of which 151 underwent MRI. Overall, 61 fetuses (40.4%) had a normal fetal brain MRI, while 90 (59.6%) presented at least one cerebral morphological anomaly, including one major cortical malformation significantly altering fetal prognosis. The use of WHO charts would reduce MRI indications for structurally isolated macrocrania by 36% without compromising diagnostic yield.

In this selected cohort, isolated fetal macrocrania was frequently associated with additional cerebral findings on MRI; most of these appeared to be minor and did not modify perinatal management. MRI may nonetheless contribute to the assessment of cerebral parenchyma when ultrasound thresholds are exceeded. In our cohort, applying WHO charts would have led to fewer MRI referrals suggesting that the use of evidence-based biometric standards could help to optimize clinical resource utilization.