Visualization of the Fetal Corpus Callosum in Routine Second Trimester Screening Ultrasound Examinations

The primary aim of this study was to determine the visualization rate of the corpus callosum (CC) in the mid-sagittal plane of ultrasound examinations performed between 18 and 22 weeks' gestation. The secondary aims were to compare this rate to that achieved 5 years earlier at the same center; assess the need for transvaginal ultrasound; identify fetal head positions most favorable for sagittal CC assessment; and evaluate the impact of maternal body mass index (BMI) and gestational age on CC visualization.

A retrospective analysis of images from second trimester screening examinations of singleton pregnancies performed between January 2019 and June 2019 was undertaken. The mid-sagittal image of the CC was scored on a scale from 0 to 4, with one point assigned for the visualization of each anatomical part. A score of 4 indicated complete visualization, a score of 1 to 3 was considered partial visualization, and a score of 0 represented nonvisualization. The chi-squared test was used to compare the visualization rates to those achieved 5 years earlier and previously reported by our group.

A total of 999 second trimester screening examinations were included in the study. Complete CC visualization significantly improved from 71.3% at initial protocol implementation and training to 92.1% in the current study (P < .001). A transvaginal approach was required in 4.4% of cases. The craniocaudal fetal head position had the highest success rate (98.2%) for CC visualization. The odds of nonvisualization of the CC were 2.6 times higher before 20 weeks (7.4%) than at or after 20 weeks (3.0%) (odds ratio = 2.63, 95% confidence interval [1.20-5.76], P = .012). Higher maternal BMI (≥30.0 kg/m²) was associated with increased rates of nonvisualization in obesity classes I-III.

Complete visualization of the CC in the mid-sagittal plane is achievable in routine second trimester morphology ultrasound examinations. Improvement in the quality of imaging is possible with a standardized protocol and training.