Unfortunately, the
Britax convertible seats do not fit newborns well at all. If you choose to bypass the
infant carrier, then I would look at a
convertible that has a lower set of harness slots to start your child off in. The Cosco Scenera fits most newborns really well.
The
BRitax seats ARE rated for 5lbs and up, but their bottom slots are 10" and that is usually about 3" too tall for them.
I would possibly reconsider getting him/her an
infant carrier.