9999.99% of the time, adoptions are irreversible. The only time they can be overturned is if some massive fraud has occurred. So I would not worry about this one ounce--it just does not ever happen. That goes triple for adoptions from foster care, where birthparents' rights are terminated involuntarily. Everyone is VERY, very careful to make sure the law is followed to the letter. So please, don't even let this cross your mind---it's like worrying about dragons or trolls or something, it's so mythical and so unlikely to happen.
Open adoptions, on the other hand, happen all the time. Even from foster care. They aren't co-parenting at all. I'm my son's mom, and I make the decisions for him. I don't consult his bmom or his bdad before I make decisions. There are varying degrees of openness, but for us, openness means occasional telephone calls and letters, swapping pictures of my son and his half siblings, and every so often talking to his bgrandma, too. If his mom were living nearby, I'd be fine with the occasional visit. And that is IT. It's not coparenting at all.
I really encourage everybody to read the best book about adoption ever, which really explains open adoption and why it is such a good idea for kids. The book is called "Family of Adoption," and it's by Joyce Maguire Pavao. It's a book that just gets rid of all the myths and fears in such a compassionate way that it helps everyone in the adoption triad come together to do what is best for kids. This book is the best ten bucks you will ever spend---it will change you, your children, and your family in so many terrific ways.
Here's the Amazon link, if you are interested:
Amazon.com: The Family of Adoption: Completely Revised and Updated: Joyce Maguire Pavao: Books