What is the basis for the formation of Omphalocele?

Omphalocele is protrusion of abdominal contents through the umbilical ring. It is due to failure of physiological hernia to reduce into the abdominal cavity , which results in non-closure of the umbilical ring. An Omphalocele may contain stomach, liver and intestines. And these contents are enclosed in a sac. Omphalocele is usually associated with other congenital anolamies (Eg; trisomy 13, trisomy 18, or Beckwith- Widemann syndrome) and with increased levels of a-fetoprotein. Ref: Gray&;s Anatomy 41st edition Pgno: 1081