I don't know if this will help or hinder with the worrying, but my cousin and his partner were due their little boy in four weeks time and he came four weeks ago at 32 weeks. He was 3lb something at birth, just over 4lb now and has been home for a week. He's unbelievably tiny (smaller than a coconut I'd say) but perfectly formed and functional, he is exclusively breastfeeding (she resisted introducing a bottle in the hospital despite pressure to do so) and doing really well. :)