A 32-year-old woman at 20 weeks gestation undergoes routine anatomy scan. The sonographer identifies a midline mass in the lumbosacral region with both cystic and solid components. There is no associated spinal dysraphism. The radiologist notes the mass is intimately related to the coccyx and sacrum. Based on the embryological origin from the primitive streak and caudal eminence, what is the most appropriate next step in management?