In the early days of microwaves, a small number of babies were hurt by being fed milk that had been overheated (case reports: Sando 1984, Hibbard 1988). Public health authorities now universally recommend against all forms of microwaving milk for babies:

FDA:

Never use a microwave oven for heating infant formulas. Microwaving may cause the bottle to remain cool while hot spots develop in the formula. Overheated formula can cause serious burns to your baby.
CDC:
If you do decide to warm the bottle, never use a microwave. Microwaves heat milk and food unevenly, resulting in "hot spots" that can burn your baby's mouth and throat.

NHS:

Never warm up formula in a microwave, as it may heat the feed unevenly and burn your baby's mouth.

Australia's Department of Health and Aged Care:

Do not use a microwave to heat up formula as this can make the milk too hot and can burn baby's mouth.

The baby formula label:

Warning: do not use microwave to prepare or warm formula. Serious burns may occur.

This is all nuts, and they should stop. The microwave is great way to heat the water for making formula because it's fast and efficient. Instead of telling people never to do this we should be giving the small amount of guidance necessary to do it safely. Ideally the guidelines for preparing infant formula using the microwave would look something like:

Heat the water, taking care not to overheat. A 6oz bottle in a typical microwave requires approximately 15 seconds to warm from room temperature, or 30 seconds from the fridge. Add the powder, cap the bottle, and shake thoroughly. Test a drop on the back of your hand: it should not feel hot or even very warm.

As long as you're heating the water before adding formula you can't get hot spots, because you still have to shake after adding the powder. And testing on your hand ensures it's not too hot.

Microwaving mixed formula or breast milk is probably also fine, but there is at least a little more reason to worry there: if you get it too hot you'll likely lose some of the nutritional value. Exactly how hot it has to get before this is a problem isn't something we know, though what studies there are indicate gentle heating is fine (Fay 1991, Sigman-Grant 1992, Ovesen 1996, Sierra 2001).

I wish public health authorities would cite their sources and give background. It needn't be in the primary public-facing documentation, but each claim ("never use a microwave") should have a link to their reasoning. I think in this case if they needed to explain their reasoning it would be clear that it doesn't hold up, and they should change their recommendations.

(I've written about this before, but only in the context of breast milk. I think the case is strongest for heating water for formula because there's no question of nutritional degradation.)

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Remember, this is a CYA aimed at overworked and sleep deprived parents with limited reasoning skills and so has to be idiot-proof. "Mix well after heating in a microwave and check the temperature on the back of your hand" would do the trick in normal circumstances, but is easy to forget if you are in a rush, or can't keep your eyes open, or are distracted by the two older kids fighting. I assume that the recommendation not to use a microwave is ignored by most parents though.

One nice thing about the microwave is that when you're doing the same thing you always do it's very consistent, so parents skipping the check when rushing through something they do all the time isn't worrying.

Well, yes, but also it is easy to forget to stir before checking, and then it feels normal on your hand, but may have pockets of hot liquid still. Not very likely, but the warnings are not intended for likely cases, more for edge cases.

As long as you put powder in after heating, which they could recommend, it's not easy to forget to shake, though? Unshaken powder with water wouldn't even come out the nipple.

Yeah, with the powder it's a no-brainer. I remember the actual liquid formula packs that needed to be kept in the fridge once open.

That's a charitable interpretation. But the steps for microwaving and not microwaving formula are the same, just in a different order. If you forget to check the temperature of overheated formula, you may burn your baby, regardless of the heating method.

It'd be like recommending against making instant oatmeal in the microwave...but you just need to heat the water separately!

The instructions would be simpler and more general this way - also apply to water heated in a kettle or boiler.

Testing with your hand or tongue is also the advice I heard from midwifes/nurses.

Isn't this simply because litigations happened and they just have to?
It's a bit dark, but I believe most warning labels aren't for our safety per se, but for the companies'.

That could explain the warning label on the American formula, but I don't see how it accounts for various national health agencies saying it?

  1. Humans who are not concentrating are not general intelligences.
  2. It's a warning label, not a categorical imperative.
  3. The warning explains why, so anyone who chooses to think about it can figure out what precautions to take, if they chose to disregard the warning.
  4. Sometimes inexperienced people become parents and/or babysitters. Everyone has to learn about uneven microwave heating at some point - better from a written warning than a trip to the ER.

It's not just the warning label, though. Even the public health sites that have multiple dense paragraphs on proper treatment of formula stick with "don't microwave it, ever".

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