Once a bottle of formula or pumped breast milk has been warmed it should be used within one hour and should never be reheated. Bacteria can grow in milk that has been left warm for too long and this could make your baby ill.
When deciding how to feed your baby, it's worth considering that the introduction of partial formula feeding will reduce your supply of breast milk.
You should never leave your young baby unattended with a bottle in their mouth – this could lead to choking. Always hold the bottle for your newborn baby while feeding.