Hirschsprung disease (HD) develops before a child is born. Normally, nerve cells grow in the baby's intestine soon after the baby begins to grow in the womb. These nerve cells grow down from the top of the intestine all the way to the anus. With HD, the nerve cells stop growing before they reach the end.
No one knows why the nerve cells stop growing. However, we do know that HD is not caused by anything the mother did while she was pregnant.
This answer is based on source information from the National Institute of Diabetes and Digestive and Kidney Diseases.