Determinants of successful midsagittal visualization of the fetal corpus callosum using 2D transabdominal ultrasound

To evaluate factors influencing visualization of the fetal corpus callosum (CC) during mid-trimester ultrasound examinations, with particular focus on gestational age, fetal presentation, and maternal body mass index (BMI).

This observational study included 860 singleton pregnancies undergoing mid-trimester ultrasound examination between 20 + 0 and 28 + 0 weeks' gestation. Fetuses with central nervous system or major structural anomalies were excluded. Assessment of the corpus callosum was performed using two-dimensional (2D) transabdominal ultrasound. Visualization was defined as clear identification of the corpus callosum in the true midsagittal plane. Associations between CC visualization and gestational age, fetal presentation, and maternal BMI were analyzed using chi-square tests and multivariate logistic regression.

The corpus callosum was successfully visualized in 748 of 860 examinations (87.0%). Visualization rates decreased significantly with advancing gestational age (p < 0.001). Successful visualization was lowest in cephalic presentation (79.8%) and highest in breech presentation (97.7%) (p < 0.001). Maternal obesity (BMI ≥ 30 kg/m²) was associated with reduced visualization rates compared with BMI < 30 kg/m² (80.2% vs. 89.5%, p < 0.001). In multivariate analysis, breech presentation remained an independent predictor of successful visualization compared with cephalic presentation (adjusted OR 9.35, 95% CI 4.25-20.57, p < 0.001). Maternal obesity independently reduced the likelihood of visualization (adjusted OR 0.49, 95% CI 0.32-0.76, p = 0.001). Each one-week increase in gestational age was associated with a 15% decrease in visualization likelihood (adjusted OR 0.85, 95% CI 0.78-0.92, p < 0.001). Gentle hand manipulation was required in 423 examinations (49.2%) and was significantly more frequent in cephalic presentation than in breech or transverse presentation (62.7% vs. 30.2% and 39.0%, respectively; p < 0.001).

Transabdominal ultrasound enables midsagittal visualization of the fetal corpus callosum in the majority of mid-trimester examinations. Fetal presentation, gestational age, and maternal obesity exert a significant influence on visualization rates; in particular, maternal obesity and increasing gestational age are associated with a reduced probability of successful assessment. Gentle hand manipulation was frequently required, highlighting the technical challenges of midsagittal corpus callosum imaging during routine examinations.