a: Lateral view of a 4-week embryo showing the relationship of primordial gut to yolk sac. b: Drawing of median section of the embryo showing the early digestive system and its blood supply.

Two L.A. couples gave birth to each others' babies after a mixup at their IVF clinic, parents Daphna and Alexander Cardinale tell PEOPLE in this week's issue ... couple's embryo into Daphna ...