New reference charts for fetal ultrasound corpus callosum length with emphasis on the third trimester

Objectives: To provide new prospective 2D ultrasound reference charts of fetal corpus callosum (CC) length on a large sample size with emphasis on the third trimester of pregnancy and to establish other standards of CC growth evaluation (external cranial occipitofrontal dimension (ECOFD) / CC length ratio and head circumference (HC) / CC length ratio) in a large population of healthy fetuses.

Methods: A prospective observational study was conducted in a single expert center for fetal ultrasound between November 1st 2021 and June 30th 2022. CC measurement was performed in all fetuses examined between 17 weeks and 36⁺⁶ weeks. Image quality criteria for a strict mid-sagittal plane of the fetal brain and caliper position for CC measurement were defined prior to data collection and only high-quality measurements were included for analysis. Fetuses with inaccurate gestational ages and at high-risk of central nervous system anomalies were excluded.

Results: Among 3591 CC measurements available, 3191 were included in this study. An accurate high-quality measurement was obtained in 92.8 % of cases. We established the third-degree polynomial model expressing the length of the corpus callosum as a function of Corpus callosum length in mm=0.00213x(GA in weeks)³ - 0.2538x(GA in weeks)² + 10.5897xGA in weeks -108.8556 +/- SD (SD=0.0567xGA In weeks + 0.1054), with an R² adj of 0.94. ECOFD/CC and HC/CC ratios were stable throughout pregnancy at 2.7 ± 0.2 and 7.7 ± 0.6 respectively.

Conclusion: These new reference charts were established using a uniform methodology of the highest quality in order to assess CC growth accurately and help clinicians correctly define a "short" CC. ECOFD/CC and HC/CC ratios may be used as additional markers of normal CC development in borderline cases.