What Should a Baby Wear Under a Swaddle?

A two-pack of bamboo muslin swaddles rolled together, one rust brown and one printed with carousel horses

This is a question about temperature, and it's one of the few places where a fabric choice is a safety choice. I write the specification for these swaddles, so I'll be direct: one thin layer, and in a warm bedroom often nothing but the nappy. The point almost everybody misses is that the swaddle itself is a layer. A muslin feels like nothing in your hand, but once it's wrapped twice round her body it's as warm as a full layer. And she can't change what you chose: she can't kick off a blanket, push an arm out of a sleeve or wriggle to a cooler part of the mattress.

Whatever you dressed her in an hour ago is what she's stuck with, and that is why it's a safety question.

By room temperature

Every row below already counts the swaddle as a layer. Forgetting to count it is where most overdressing starts.

Under-swaddle layers for a single bamboo muslin wrap
Room	Under the swaddle	Notes
29-31°C, no air-con	Nappy only	Consider skipping the wrap on the hottest nights
27-28°C, fan only	Nappy, or a sleeveless bodysuit	Watch the back of the neck for dampness
25-26°C, air-con	Short-sleeve cotton or bamboo bodysuit	The most common Singapore bedroom
23-24°C, air-con high	Long-sleeve bodysuit	Rare here; raise the thermostat instead if you can
Any	No hat indoors	Babies lose heat through the head: leave it uncovered

Elsewhere the rule is one more layer than an adult. Here it is about the same as an adult, or one fewer.

How to tell she is too warm

Two fingers on the back of her neck, or a hand on her chest between the shoulder blades.

Warm and dry is right. Damp, clammy or hot means one layer too many.

Don't judge by her hands or feet. They run cool in almost every baby and aren't a temperature signal.
A flushed face, damp hair at the nape, or rapid breathing during sleep all point the same way.
Heat rash on the neck, chest or back of the shoulders is the delayed version of the same message.
If she has wriggled out of the wrap and calmed down straight away, she was probably too warm.

If heat is the problem

Does the swaddle fabric change the answer?

A good deal. An open bamboo muslin weave lets air through and draws moisture away from the skin. A jersey wrap, a double-layer cotton or anything quilted traps both. Two wraps with the same description on the label can differ by a full layer of warmth on a humid night.

That's why I specify bamboo for our swaddles, and I say so at every opportunity. In a climate where the fabric has to keep her cool because she can't do it herself, the weave is more than a matter of comfort.

Mentioned above

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On fabric

What she should never wear under one

It's a short list, and everything on it is there for the same reason: overheating.

A hat, indoors. Head covering plus a wrap is a well-established overheating combination.
Fleece, wool or anything padded. There's no bedroom in this country that calls for it.
A second swaddle over the first. If she seems cold, raise the room temperature and don't add layers she can't remove.
A sleep sack under or over the wrap. Choose one format, not both.
Anything with a hood, ties, ribbons or loose fastenings that could ride up toward her face.

Once the swaddling weeks are over, the same thinking applies to the sleep sack, and the TOG rating does the sums for you. We've covered sleepwear in full for both stages, including what changes when the air-con goes off at 6am and the room warms up eight degrees before she wakes. Eight is what we measured. I'm not certain it's the same in your home.

Evidence

This guide is general information, written to help you understand a topic and ask better questions. It is not medical advice and does not replace your own doctor, midwife or paediatrician. Anything that worries you about your baby belongs with them.

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