My son Parker didn't gain his birth weight back for about 4 weeks too, we were not allowed to leave hospital till he was almost 2 weeks. He ended up being diagnosed with reflux at 6 weeks, so looking back I think that was part and parcel with that. If it helps any he is at the lower end of normal weight now (he is almost 3) but he is more than on target for all developmental skills.

Good luck with the doctors appointment.