First-trimester cavum veli interpositi: prevalence and natural history

To confirm the identity and assess the prevalence and evolution of the fluid-filled interhemispheric midline structure, thought to be the cavum veli interpositi (CVI), in fetuses at 11-14 weeks' gestation.

This was a retrospective study of first-trimester ultrasound scans performed at a single center over 3 months. Inclusion criteria were singleton pregnancies at 11-14 weeks' gestation with known neonatal outcome. Five experts reviewed the images. Mixed-effects logistic regression and generalized estimating equations (GEE) were conducted to analyze the associations between the presence of the structure and variables including ultrasound approach (transabdominal vs transvaginal), maternal body mass index (BMI), gestational age, fetal crown-rump length (CRL) and biparietal diameter (BPD). Second-trimester ultrasound scans of the fetal central nervous system at 18-24 weeks' gestation were evaluated for the persistence of the CVI in fetuses in which the structure was observed in the first trimester.

Of the 223 cases reviewed, 104 were included, among which the CVI was observed in 25 (24%) cases. There was no statistically significant difference in CVI visualization between transabdominal and transvaginal ultrasound examinations. GEE showed significant associations between the presence of the fetal structure and CRL (odds ratio (OR) per 10-unit increase, 1.32; P < 0.0001) and BPD (OR per 10-unit increase, 1.88; P = 0.0011). Maternal BMI and gestational age showed no significant effect on the presence of the CVI. At second-trimester follow-up of the 25 fetuses in which the CVI was observed initially, 44% still showed a CVI, 32% exhibited a cavum vergae, 4% had both structures and 20% had neither.

Based on its anatomical location and, in some fetuses, its visualization as a distinct entity from the third ventricle, the identity of the interhemispheric midline structure in the suprathalamic region of the fetal brain between 11-14 weeks' gestation was confirmed as the CVI. The CVI and/or cavum vergae persisted into the second trimester in 80% of fetuses identified initially as having a CVI. Its presence is not linked to pathology, offering reassurance to practitioners and parents. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.