Prenatal ultrasound at 20 weeks revealed a midline mass that appeared to contain intestines and was membrane bound. Probable diagnosis is:

The most likely diagnosis is an omphalocele resulting from a failure of herniated bowel to return to the abdominal cavity at 10 to 12 weeks of gestation. Because the bowel normally herniates into the umbilical cord, it is covered by amnion. This situation is in contrast to gastroschisis, in which loops of bowel herniate through an abdominal wall defect and are not covered by amnion. Ref: Langman's embryology 11th edition Chapter 14.