Kaya was 9lb1oz at birth but at 6 weeks was only about 10lb. My HV also started worrying but I knew that she was fine and ignored her for the most part - my only concession was to see the GP like she asked and he said that as long as she seemed healthy and happy, wasn't sleeping too long and wasn't sleeping too little then there was nothing really to worry about. He also said that mum's know their babies best and if they don't think there's something wrong then there usually isn't.
We then stopped taking Kaya to the HV and bought a set of digital scales so we could monitor her weight ourselves. We found that her weight could vary by as much as a pound depending on if she had just been fed, just had a wee or a poo, what time of day it was etc.
Kaya is breast fed.