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 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) - 0.2538x(GA in weeks) + 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.