As a fetus is growing in the mother’s uterus before birth, different organ systems are developing and maturing. Between the 6th and the 10th weeks of pregnancy, the intestines actually project into the umbilical cord as they are growing. By the 11th week of development, the intestines should return to the abdomen. When the fetus is growing and developing during pregnancy, there is a small opening in the abdominal muscles that the umbilical cord can pass through, connecting the mother to the fetus. As the fetus matures, the abdominal muscles should meet in the middle and grow together, closing off this opening. An omphalocele occurs when the abdominal organs do not return to the abdominal cavity as they should.
It is not known what causes omphaloceles. Steps that normally happen in the development of the abdominal organs and muscles simply did not happen properly. It is not known to be caused by anything the mother did during pregnancy. Source: Children”s Hospital of Pittsburgh