The fact underneath every rule

Powdered infant formula is not a sterile product. The NHS puts it plainly: even sealed tins and packets "can sometimes contain bacteria". The HSE says the same. The organism everyone is worried about is Cronobacter, which the CDC describes as living in the environment and in dry foods such as powdered infant formula, and which causes infections that are rare but serious in babies.

Every preparation rule that follows exists to deal with that one fact. The rules differ by country because the two health systems weigh the same risk differently.

The divergence, named

The UK and Irish position: 70C, every feed

The NHS instructs you to boil at least a litre of fresh tap water, then leave it to cool in the kettle for no more than 30 minutes, so that it remains at a temperature of at least 70C. Water at that temperature kills harmful bacteria in the powder itself. Ireland's HSE gives an identical instruction, in the same words, with the same 30-minute window.

Both trace back to the joint WHO and FAO guidelines on safe preparation, storage and handling of powdered infant formula, which set out the reconstitution temperature, cooling, holding and feeding times that these national services adopted.

The US position: boiled water only for some babies

The CDC's line is different. For most babies, most of the time, it says it is safe to prepare powdered formula "by mixing it with tap water and following the manufacturer's instructions on the container", filtered or unfiltered. Extra steps apply only to babies who are younger than two months, were born before 37 weeks, or have a weakened immune system. For those babies, the CDC says to boil the water and then wait about five minutes before mixing.

The AAP echoes this: boiled water should be used "for infants younger than 2 months of age, those who were born prematurely and those who have a weakened immune system", and after boiling, let it cool for about five minutes.

Both positions are defensible. The UK and Ireland treat every feed as a potential exposure. The US targets the babies for whom an infection would be most dangerous. If you are following UK or Irish guidance, use 70C water for every feed; if you are in the US, follow the label unless your baby is in one of the three named groups.

What everyone agrees on

Beneath the disagreement there is a large shared core.

Water first, powder second. The NHS, the CDC and the AAP all say to measure the water into the bottle before adding powder. The AAP calls it a key point: "always add the powder to the water that is in the bottle, not the other way around".
Never change the ratio. Extra powder can make a baby constipated or dehydrated. Extra water dilutes the nutrition. The AAP describes over-dilution leading to water intoxication, disturbed electrolyte balance and, in severe cases, seizures.
Level the scoop, and use the right scoop. Use only the scoop supplied with that tin, levelled with the flat edge of a clean dry knife or the leveller provided. Different formulas come with different scoops. The HSE gives the underlying ratio for its market: one level scoop per 30ml of water, so a 90ml feed takes three scoops.
Nothing else goes in. No sugar, no cereal, no medicine unless a health professional has told you to.
Never microwave. Microwaves heat unevenly and create hot spots that can burn a baby's mouth and throat. Cool a hot feed under cold running water with the lid on, and test a drop on the inside of your wrist. It should feel warm or lukewarm, never hot.

The sequence, in order

Following UK guidance, the order runs like this. Fill the kettle with at least a litre of fresh tap water that has not been boiled before. Boil it, then start the 30-minute clock. Clean and disinfect the surface you will work on and wash your hands. If you have used a cold-water steriliser, shake off the excess solution or rinse with cooled boiled water from the kettle, never tap water. Stand the bottle on the disinfected surface and pour in the exact amount of water the manufacturer specifies, while it is still hot. Add the levelled scoops. Fit the teat by its edge into the retaining ring, screw it on firmly, cap it and shake until the powder dissolves. Cool the capped bottle under cold running water, test the temperature on your wrist, feed, and throw away whatever is left in the bottle afterwards.

Water: which kind

Use fresh tap water. The NHS says not to use water that has been boiled before, and not to use artificially softened water. Bottled water is not usually recommended, because it is not sterile and may contain too much sodium or sulphate; it is a fallback for when the drinking supply is unavailable or contaminated. The HSE adds that if you do use bottled water you should still boil it, use still rather than sparkling, and avoid anything labelled natural mineral water.

Machines, and where the two services part again

The NHS permits formula preparation machines provided the water reaching the powder is at least 70C, and suggests checking with a digital food thermometer; if it is not, stop using the machine. The HSE takes the opposite view and does not recommend preparation machines or bottle warmer machines at all, on the grounds that there is not enough research to show they are safe and effective. The HSE also advises against UV sterilisers for the same reason.

Storage, and the three clocks

The safest habit, everywhere, is to make each feed fresh as your baby needs it. Where a feed has been made in advance, the limits differ slightly. UK guidance allows 24 hours in the fridge, four hours in a cool bag with an ice pack, and two hours at room temperature. The CDC allows prepared formula 24 hours in the fridge and says to use it within two hours of preparation and within one hour of the start of a feed. The AAP says prepared formula must be discarded within one hour after you have started feeding it, and that an opened container of ready-to-feed or concentrate should be covered, refrigerated and discarded after 48 hours.

Whatever the clock, leftover milk from a feed your baby has drunk from goes down the sink. Saliva in the bottle lets bacteria grow.

When something is wrong

Speak to a midwife, health visitor or GP if your baby is unusually sleepy or floppy, is refusing feeds, has diarrhoea or vomiting, or has a fever. Infections from contaminated powder are rare, but they are the reason these rules are as strict as they are.

Making Up Formula Safely