I remember having this fear when I had my first son. Before I went into labor I told my husband to stay with the baby, no matter what. Once my son was born, they cleaned him up and of course, put identification bands on him. My husband never left his side. At that point, unless someone changes the bands, you should have no doubt. Besides, once you see your baby ... you can't be fooled, you'll know. If it's already after the fact and you have any doubt at all, there's always DNA testing.