I have done holding a spot for 5 months. I got a non refundable deposit and signed contract although that doesn't obligate them to start with me.
I guess you have to be sure about the family and whether you are willing to wait and whether you will be okay financially if it doesn't happen or the family backs out at the 11th hour.
I also wanted to make sure the child seemed to have a good temperment from the get go. Nothing sounds scarier than holding a spot and then it doesn't work out because the parenting style and the child's temperment are not a match.
I am not sure about this whole securing a daycare before the baby is even born though. That seems a little far out there. lol