I had been expressing my milk but eventually gave up due to having mastitis twice. My breast milk runs out shortly and we are not sure which is the best formla to use. Boys were born at 31 weeks and are doing really well, now wight 5lb7 and 5lb2, they are coming home on Sunday. Both feeding really well about 100ml every 4 hours. Not sure which would be the best formula to use. Been told we need to use cartons first as they are more sterile than powder formula? Cartons cost so much more than the powder, was anyone else told this? Any advice appreciated.