Where an unborn fetus is expelled from the decomposing body of the dead mother through the birth canal by a build up of the gasses of putrefaction in the abdominal cavity. This can only take place when the body has not been embalmed. This process can take weeks, or months, the time frame being temperature dependant.

http://www.drgreene.com/21_1458.html
http://abclocal.go.com/kgo/news/041603_nw_missing_bodies_found_experts.html