A 32-year-old primigravida presents for routine antenatal ultrasound at 18 weeks gestation. Sonography reveals a single live intrauterine pregnancy with normal biometry for dates. However, the fetal spine demonstrates a midline defect in the lumbosacral region with protrusion of neural tissue and cerebrospinal fluid. The lateral ventricles measure 12 mm. What is the most appropriate next step in management?