Case Study: Borderline Fetal Ventriculomegaly on a Technically Difficult Mid-Trimester Scan

Case at a Glance

A 31-year-old primigravida patient presented for a routine 20-week anatomy scan. The scan was technically challenging due to a persistent suboptimal fetal position. A borderline measurement of the left lateral cerebral ventricle (9.7 mm) was reported, prompting patient anxiety and a need for a follow-up assessment.

Patient's Story

The patient, a 31-year-old woman in her first pregnancy, presented at 20 weeks and 4 days of gestation for her routine fetal anatomy ultrasound. Her pregnancy had been unremarkable up to this point. During the scan, she was informed that the fetus was in a difficult position for visualization. After a prolonged 90-minute examination, the sonographer was still unable to obtain clear images of the fetal face. The patient later received the report, which noted a borderline measurement in the fetal brain, causing her significant anxiety. She was concerned about the finding itself, a discrepancy in the reported measurements, and the recommended 3-week waiting period for a follow-up scan.

Initial Assessment

A routine mid-trimester fetal ultrasound was performed. The study was noted to be technically difficult due to a persistent cephalic fetal position, with the fetal head deep in the maternal pelvis and the fetal face oriented posteriorly, away from the transducer.

Key findings from the report included:

• Fetal Biometry: Biparietal Diameter (BPD) of 4.9 cm and Head Circumference (HC) of 18.5 cm, both consistent with 20-21 weeks gestation.
• Intracranial Anatomy: Incomplete visualization of the fetal face and profile. The right (non-dependent) cerebral ventricle was suboptimally visualized but appeared to be within normal limits. The dependent left lateral ventricle measured 9.7 mm at the atrium, placing it at the upper limit of the normal range (<10 mm).
• Measurement Discrepancy: The formal report listed the measurement as 9.7 mm, while a static image provided to the patient was annotated with a measurement of 9.0 mm. This discrepancy was attributed to the technical difficulty of obtaining a true axial plane, with the 9.7 mm representing the maximum dimension obtained during real-time scanning.

The Diagnostic Journey

The primary diagnostic challenge was differentiating true borderline ventriculomegaly from a measurement artifact caused by suboptimal fetal positioning. The oblique angle of insonation of the dependent ventricle can lead to a tangential measurement, artificially increasing its apparent size. The discrepancy between the 9.0 mm and 9.7 mm measurements further supported the likelihood of measurement variability in a difficult scan.

The patient was counseled by her obstetrician. It was explained that while 9.7 mm is at the threshold of normal, it is not diagnostic of ventriculomegaly, which is typically defined as a measurement of 10 mm or greater. The rationale for a 3-week follow-up was provided: it allows time for fetal growth and, crucially, for the fetus to change position, which would hopefully permit a more accurate and definitive assessment. Pushing for an earlier scan was deemed unlikely to be helpful, as the fetal position might not have changed sufficiently to improve visualization.

Final Diagnosis

Provisional Diagnosis: Technically limited 20-week anatomy scan with a borderline measurement of the left lateral cerebral ventricle (9.7 mm), requiring follow-up to rule out mild ventriculomegaly.

Treatment Plan

The management plan focused on clarification and reassurance.

1. Counseling: The patient was counseled on the nature of the borderline finding, the high probability of it being a normal variant or a result of scan limitations, and the management plan.
2. Follow-up Imaging: A repeat, detailed fetal ultrasound was scheduled for three weeks later, at approximately 24 weeks gestation.
3. Contingency Planning: The patient was informed that if the follow-up scan confirmed ventriculomegaly (≥10 mm), a referral to a Maternal-Fetal Medicine (MFM) specialist for further evaluation, possibly including a fetal MRI or amniocentesis, would be recommended.

Outcome and Follow-up

The patient returned at 23 weeks and 5 days for her follow-up ultrasound. At this scan, the fetus was in a more favorable transverse position, allowing for excellent visualization of all intracranial anatomy.

Careful re-measurement of the lateral ventricles was performed in the correct axial plane. The left lateral ventricle measured 7.5 mm and the right measured 7.2 mm, both well within normal limits. The remainder of the fetal anatomical survey was completed and was unremarkable.

The initial borderline finding was resolved and attributed to a measurement artifact from the challenging fetal position at the 20-week scan. No further specialized imaging or MFM referral was required. The patient's anxiety was alleviated, and the remainder of her pregnancy proceeded without complications. She delivered a healthy male infant at 39 weeks gestation.