When is it safe to change from pram (baby flat) to pushchair (baby sitting?)

[gerbo]

I have a lovely 11 week old daughter and she likes to look around her - when is it safe to change her pram (Silver Cross 3D travel system) from the 'pram' mode we have at the moment where she lies flat, to 'pushchair' mode where she sits up? She can't quite hold her head steady yet - is it too soon? Need advice!

I'd wait until she can support ehr head before going very upright but it depends if you have a slight angle incline on the pushchair and it can still go flat for naps in which case it is probably ok whenever. ds was about 4 months.