A 28-year-old primigravida at 18 weeks gestation undergoes routine ultrasound screening. A fetal anomaly is detected: the fetus has a large midline defect in the anterior abdominal wall with herniation of bowel loops and liver into the amniotic sac. The defect measures 8 cm and is not covered by peritoneum or skin. Based on the embryological origin of this defect, what is the most appropriate next step in management?