We used our B-Agile from birth. The back of the seat lays down to ALMOST flat. Its about a 10 degree angle. We just strapped LO in and used it like that. He would stretch out and nap, he loved it. I believe the first time we took him out in it was at 2 weeks.
I can't say enough good things about the B-Agile. its a great stroller. You can but an adapter bar for it and use it with other carseats you know! You don't have to just use the Britax carseats (however, I LOVE our B-Safe as well!)
Good luck OP! and congrats!