My baby was born with imperforate anus? and died after surgery. he did not have anal opening. can you tell me what caused this disease to have formed?

This is not a disease, but a genetic deformity which persists erly in the development while in the womb. This condition needs urgical repair. You might find this article helpful: https://www.nationwidechildrens.org/conditions/anorectal-malformations-or-imperforate-anus

Dr. McLeod has it right. During fetal gut formation in the womb, babies tube-like gut moves down to where a dimple in the anal skin will meet it and dissolve. The birth defect can vary from a very low thin membrane that can be opened simply, or the gut may stop quite a distance from the bottom. Geneticists have suggested a random pattern for most. I would consult a clinical genecist for your info.