The midwife is correct. It is now believed that bottles used for breastmilk do not need to be sterilised - just washed in hot soapy water - due to the properties of the milk. However, formula bottles do need to be washed and sterilised properly due to potential growth of bacteria.

When I was expressing, we only sterilised the pump once a week (washed after every use of course!) but all bottles were washed and sterilised as I couldn't tell which ones were used for breast milk and which were formula (we were comp feeding).