You should never make formula with breastmilk instead of water.
While breast milk is the gold-standard in infant nutrition, so it might seem to make sense, formula is not designed to be mixed with anything other than water.
Formula powder is made using milk proteins combined with nutrients like essential fatty acids, vitamins and minerals to make it closer to breastmilk. Water is the only thing taken out – and that’s the only thing you need to add when you make up your baby’s bottle.
If you use anything other than water (including breastmilk) you could cause dehydration and even kidney problems. You also risk upsetting the balance of nutrients your baby gets. Every scoop of formula is carefully designed to provide just the right amount – adding breast milk could mean they get too much of some vitamins or minerals, which can be dangerous.
But if you want to mix-feed – giving your baby some breastmilk and some formula – you can. Many moms choose to do this if they’re struggling with breastmilk supply, to give them some flexibility with feeding or when they’re ready to start weaning off the breast.
Mixing formula and breastmilk – one bottle or two?
While you should never replace the water you use to make formula with breastmilk, you can combine breastmilk and formula made with water in a single bottle.
If you know that your baby takes four ounces per bottle, you can simply make up a two ounce bottle of formula, and top it up with two ounces of breastmilk. This works well if you know that your baby is a consistent feeder and is likely to finish a full bottle.
If your baby doesn’t always finish their feeds, it might be better to give them formula and breastmilk in two separate bottles. Feed them the breastmilk bottle first, burp them and then offer the formula. This is more hassle (and more washing up and sterilizing!), but it means you won’t waste breastmilk.