We did something similar except we didn't even tell my mother we were thinking about adopting--again. We were older and didn't want to hear anything negative so we just decided to wait to actually present the baby to anybody who might have cared.
I went to Vietnam by myself the weekend my youngest graduated from high school. We told my Mom, who was in assisted living, that I was going to take a long deserved trip with a few friends. The day I came home with 4 month old Sweetness, I had my DH in the hall with her in a baby carrier while I went into her room. I told her "Remember how we always said we wanted to adopt again? Well we did and she's in the hall right now." Good thing she was sitting down. Quel Surprise!
Later she told me she feared I had run off with a boyfriend! That really cracked me up.
She opened up a gift bag and learned she was a grandma. Her reaction is the best!