My little boy was born at 36 weeks (also an IVF baby). He was 2.48kg when born, which I believe is roughly the same weight as your daughter. He'd got to 8.45kg by the time he was 6 months old. He had a slow start, but then took off very quickly.

Thing is, he has been formula fed (my milk never came in) and boys grow at different rates to girls. Is she meeting developmental milestones? I believe that's more important than size - some babies are just meant to be small, and that's ok.

BW