What happens if a baby ingest its own fecees?

It is not possible for a baby to digest it's own feces because it does not produce any. Feces are a waste product that we humans release when we have sucked all the nutirents out of it. An unborn baby (as I prosumed you meant) does not digest it's own food, rather, the mother digests it for the baby. Then, most of the nutrients that the mother produces, is sent down into the baby via the umbilical cord. Therefore, the baby cannnot produce and feces, let alone digest.

If you were talking about a born baby, nothing would happen except for the baby just releasing it exactly as it was when it was eaten.