Fabrics for baby clothes: what cotton, wool and fleece do best

Each fabric has a place where it does its best work: soft cotton against the skin, wool or fleece over it for warmth, and a smooth jacket outside to keep the wind and rain off the warm layers. Those are the three layers of a cold-⁠day outfit, and on a mild day you need only one or two of them.

No. Polyester is the plastic fiber behind most of the warm and weatherproof things a baby wears: fleece, the shell of a rain jacket, the filling of a pram suit (which US shops call a snowsuit) and most warm pajamas. It takes up almost no water, so it dries fast and keeps its warmth when it gets damp, which is exactly what you want from a warm layer or a jacket.

Next to the skin, that same trait works against it. Polyester can't soak up sweat the way cotton does, so on a warm day a polyester bodysuit turns clammy sooner. That's why the pieces that touch the skin all day are usually cotton, and polyester goes on over them.

Yes. What to watch with fleece is its warmth, in bed and indoors, not the fabric itself. Fleece is polyester brushed into a soft, fluffy knit: warm for very little weight, almost impossible to ruin in the wash, and dry almost as soon as the spin cycle ends. It makes an easy warm layer under a light jacket or a stroller blanket.

Where parents get caught out is the wind. Fleece is an open knit, so a cold wind blows straight through it and rain soaks in within minutes. A fluffy bear suit looks cozy, but on a windy day the warmth blows right out of it, and a smooth jacket over it keeps it in.

How warm a piece is depends on how thick and fluffy the fabric is, much more than on the fiber. The raised pile of a fleece and the loops of a knit hold still air, and still air is what keeps the heat in. A thin microfleece and a dense sherpa pram suit are both polyester, and they're nowhere near each other in warmth.

So trust your hands more than the tag. If you can see light through a piece when you hold it up, it's a light layer, whether it says wool, cotton or fleece. Once your baby is dressed, the back of the neck tells you more than any label.

Cotton is the right fabric for nearly everything that touches a baby's skin. It's soft and cheap, it soaks up sweat, and it comes through the hot wash after a messy feed none the worse.

The catch is water. Drool, spit-⁠up or a leaky diaper: cotton soaks it up like a dishcloth and holds it against the skin, where it keeps pulling heat out of the body. So when your baby feels chilly through a wet bodysuit, a sweater on top won't fix it, because the wet layer underneath goes on chilling. Change the bodysuit first, then see whether the sweater is still needed.

Damp usually starts on the inside. A baby can look perfectly dry in a waterproof jacket while the collar is soaked with drool or the back is sweaty from the car seat. The collar looks fine from the front, so slide two fingers inside it and under the arms before you reach for a blanket. In the drooling months, a bib with a waterproof back keeps the bodysuit under it dry, where a plain cotton bib soaks straight through to the chest.

That's why the spare set in the stroller bag is a base, not a jacket: a bodysuit, leggings and socks in a zip pouch. Change the layer next to the skin, put the dry warm layers back on, and carry on.

Yes, and it's at its best as the warm layer over a cotton bodysuit. Wool can take up about a third of its weight in water before it even feels damp, and it gives off a little heat as it does. So a wool cardigan stays warm through drizzle or a sweaty nap in the carrier, where a cotton sweatshirt would turn cold and clammy.

Whether wool itches depends on how fine its fibers are. Ordinary wool can prickle, and merino is fine enough to wear on bare skin.

Pick wool that opens down the front. A cardigan slips off a half-⁠asleep baby in the carrier or stroller, and wrestling a snug wool pullover over a newborn's wobbly head in a warm café is another story.

A thin rain jacket or windbreaker adds very little warmth of its own. Its job is to keep the wind and rain off the fleece or cardigan underneath, so the still air in them stays warm. A padded pram suit is the exception, warmth and weather in one piece, and its padding is why it never goes under a car seat harness.

Most rain jackets let very little sweat out, so the moment you step into a heated shop or bus, the jacket turns into a greenhouse. Open it and take the hat off as soon as you're inside, even for a quick errand. If you're staying a while, in a café or on a long bus ride, the fleece comes off too, even if it wakes your baby.

The hangtag is there to sell. The small label sewn into the side seam says what the piece is made of: every fiber by its real name, biggest share first.

Bamboo is the clearest case. Bamboo clothes feel silky, cool and stretchy, and they're lovely on a warm day, but the fabric is rayon. The bamboo is dissolved into pulp with chemicals and spun into a new fiber, and none of the plant is left in it, so an honest label says viscose or rayon made from bamboo. Modal is the same kind of fiber, made from beech wood. Both soak up even more water than cotton and take longer to dry on the line.

A little stretch is a good reason for a blend. A cotton bodysuit with 3 to 5% spandex (elastane on European labels) goes over the head more easily and keeps its neckline through many washes. A bodysuit sold as cotton rich, on the other hand, can be half polyester, and only the sewn-⁠in label says so.

Everything a baby wears meets a leaky diaper sooner or later, so turn the piece over in the shop and read the care label. A cardigan that wants hand-⁠washing and drying flat on a towel ends up at the bottom of the laundry basket for weeks.

Cotton and fleece go in the machine with everything else. Superwash wool is treated for the machine and takes a gentle cycle. Other wool goes in the sink or on the wool cycle, because a merino cardigan in a hot wash comes out doll-⁠sized. Wash fleece without fabric softener, and turn knits inside out so they pill less.