We are involved in semi-open/closed relationships with both of our son's parents. We send pictures and updates when they request them. We have met our new baby's mom, but have never met our eldest son's parents.
When our baby turns one, we are doing an album with an update and also giving her a locket with our son's photo inside with the inscription on the back saying "Love Jesse". We asked our Social Worker if that was okay, and she said it was a perfect gift idea.
Just another idea for later on would be to collect two "First Curl Boxes" and two "First Tooth Boxes" and pass one of each to the birthmom with curls and a tooth inside. We are doing that as well.
I am sure your pbmom will appreciate anything that you give her.
