With an omphalocele, omphalo- refers to the naval, also known as the bellybutton—or more formally the umbilicus—which is the attachment site of the umbilical cord, and -cele relates to hernia or swelling.
Omphalocele, therefore, is when some of the bowels herniate out into the umbilical cord.
During the fourth week of fetal development, the embryo starts to change shape from a flat, three-layer disc to something more shaped like a cylinder, a process called embryonic folding.
In the horizontal plane, the two lateral folds eventually come together and close off at the midline, except for at the umbilicus, where the umbilical cord connects the fetus to the placenta.
This folding allows for the formation of the gut within the abdominal cavity.
During around the sixth week of development, the liver and intestines grow really quickly, and because the abdominal cavity’s still pretty small, there’s limited space, which causes the midgut to herniate through the umbilical ring into the umbilical cord, and this happens normally.
At about week 10, though, the abdominal cavity typically has grown enough to allow the midgut to come back from the umbilical cord.
With omphalocele, the midgut—along with potentially other organs from the abdominal cavity—fail to return back to the abdominal cavity, and therefore stay in the umbilical cord all the way through fetal development and even after birth.
Now, since the intestines and potentially other organs aren’t meant to be in the umbilical cord, there can be complications like the abdominal cavity not growing to its normal size, as well as pinched blood vessels and loss of blood flow to an organ.
So with an omphalocele, after birth the abdominal organs protrude out of the body, but are contained within the umbilical cord, meaning the organs are sealed by a peritoneal layer.