A newborn cannot warm herself the way you can. Here is what that means on a cold morning, when a sweater is the right answer, and what a good one is actually doing.

The thing new parents are rarely told

When you feel cold, your body does several things without asking you. You shiver. Blood pulls away from your hands and feet to keep your core warm. You move around, you pull a shawl closer, you get up and shut the window. A baby can do almost none of this.

For roughly the first year of life, and especially in the first three months, a baby's temperature control is still being built. Paediatric textbooks call it thermoregulation; the plain-English version is that a baby's body is a very small room with very thin walls and a heater that is not yet wired up properly. Newborns lose heat around four times faster than an adult does. That is the whole reason baby woollens exist. Not fashion, not tradition. Physics.

Four reasons a baby loses heat faster than you do

1. Too much surface, too little body. Heat leaves through skin. A newborn has far more skin for every kilo of body weight than an adult, so the same cold air that barely touches you is pulling warmth out of a baby from every side at once.

2. Almost no insulation. The layer of fat under the skin that keeps adults warm is very thin in a newborn, and thinner still in babies born early or small. There is nothing between the cold and the blood.

3. Babies cannot shiver. Shivering is how adults make emergency heat. Newborns do not shiver effectively. Instead they burn a special reserve called brown fat, which sits around the neck, back and kidneys. It works, but it costs them: a cold baby uses up oxygen and energy warming herself that should have gone into feeding, sleeping and growing. And once the brown fat is spent, there is no back-up. The only other things a newborn can do are curl up tight and pull blood away from the skin, which is why cold hands and feet are normal and not a reliable sign of anything.

4. A big head. A baby's head is about a quarter of her body length, and it is where a large share of heat escapes. This is why every grandmother reaches for the cap first, and why she is right.

The four ways the heat actually leaves

Doctors describe four routes, and each one maps onto something you do at home.

• Evaporation — water drying on skin takes heat with it. This is bath time. Dry the baby completely, head first, before anything else.
• Conduction — touching something cold. A cold changing mat, a steel weighing pan, a floor tile in December. Put a cloth down first.
• Convection — moving air. A fan, an open window, a draught under the door, the AC vent pointed at the cot. Still air is warm air.
• Radiation — being near a cold surface without touching it. A cot pushed against an outside wall or under a window loses heat all night even when the room feels fine to you. Move it to an inside wall.

What "cold" means for a baby, and how to check

The World Health Organization defines a newborn's normal body temperature as 36.5–37.5 °C. Below 36.5 °C is cold stress; 38 °C or above is a fever. The important part is that a baby can be in the cold zone without looking cold. She may not cry. She may simply become quiet, feed less, and sleep more than usual.

Do not judge by hands or feet; they are cool on most babies most of the time. Feel the back of the neck or the chest with the back of your hand:

• Warm and dry — she is dressed right.
• Cool — add a layer.
• Hot, damp or sweaty — take a layer off now. Overheating is its own risk, especially during sleep, which is why the answer is never "as many layers as possible". It is the right layers.

Does she always need a sweater? Honestly, no. It depends on the room.

A sweater is a tool for a temperature, not a rule for a season. The room matters more than the month. The guide most paediatric sources agree on is "one more thin layer than a comfortable adult in the same room", and it plays out like this:

• Above 25 °C (most Indian afternoons): no sweater. A cotton vest or jhabla is enough.
• 20–25 °C: usually no sweater. A full-sleeve cotton sleepsuit, or a jhabla with cotton pants.
• 16–20 °C (a Pune or Bengaluru morning, a Mumbai bedroom with the AC at 18): a cotton layer underneath, and a light sweater or cardigan on top. This is the zone most Indian babies spend their winter in.
• Below 16 °C (Delhi, Lucknow, Chandigarh nights; the hills): cotton base, sweater, and a cap and socks, with a blanket for the pram or the car.

Doctors suggest keeping the room a baby sleeps in around 20–22 °C where you can. Most Indian homes cannot control that, which is exactly why the sweater does the job the thermostat would have done.

It changes with age

Birth to 3 months is the highest-risk window: brown fat only, no shivering, thin skin, almost no fat. This is when the cap, the socks and the sweater matter most and when you check the neck most often.

3 to 6 months: the baby is putting on "baby fat", which is real insulation, and the ability to sweat and shiver is beginning. Still immature, but improving.

9 to 12 months: the internal thermostat is largely mature. A one-year-old handles a cool room much the way you do, and you can relax the rules to "same as me, plus a light layer when in doubt".

A baby born early takes longer at every stage, because most of the insulating fat and brown fat is laid down in the last three months of pregnancy. Count from the due date, not the birth date, and follow your paediatrician's advice over any general rule.

Why the sweater, specifically, and not just another blanket

A blanket keeps heat in only while it stays on, and a baby who kicks, a mother who lifts her to feed, a car-seat buckle, all undo it in seconds. A sweater stays put. It keeps the chest, the back and the belly, the places heat matters most, covered through feeding, carrying and the 3 am wriggle. A cap does the same job for the head, and socks for the feet, which are the first thing to go cold and the last thing anyone checks.

What to look for in a baby sweater

• Front-opening, so you never pull it over the head. Cardigan-style with buttons, or a set that opens flat, means you can dress a sleeping baby without waking her and get it off fast if she runs warm. Skip drawstrings at the neck, loose beads or sequins, and loose, lacy knits a finger can get caught in.
• Soft, light knit rather than heavy. Warmth comes from trapped air, not from weight. A light, tightly knitted baby yarn holds more warm air against the skin than a thick, loose one, and it does not pin the baby's arms down.
• The right fibre, worn the right way. Ordinary sheep's wool is scratchy on baby skin and can set off eczema. Fine merino is soft but expensive and shrinks in a careless wash. Good-quality anti-pilling acrylic baby yarn (what most Indian baby woollens, including ours, are knitted from) is soft, hypoallergenic, machine-washable and holds its shape wash after wash; its one limit is that it breathes less than cotton, so it belongs over a cotton layer, never directly on bare skin for long, and it comes off the moment the neck feels damp. Whatever the fibre, keep the baby away from open flames and room heaters: synthetic yarns melt, natural ones burn.
• Nothing that scratches. Babies' skin is thin. A tag at the neck or a rough seam is not just uncomfortable; a baby who is itching will not settle, and a baby who does not settle does not stay warm. Look for tag-less necks and flat seams.
• Cap and socks that match the sweater's weight. A heavy cap with a thin sweater, or the other way round, is how you end up with a baby who is sweaty on top and cold below.
• Yarn that survives washing. A newborn's sweater is washed every second day. If it pills or stiffens after a few washes, it is no longer soft against the skin, and you are back to the scratching problem.

A note on Indian winters

Parents in Delhi, Lucknow or Chandigarh already know why the cap goes on in November. The ones who get caught out are in Bengaluru, Hyderabad and Pune, where a 14 °C morning follows a 28 °C afternoon and the house has no heating. The swing, not the absolute cold, is what a baby cannot handle. A light sweater set that goes on at 6 am and comes off at 11 is exactly the right tool for that climate. So is one in Mumbai for the AC.

What we make, and why it is made this way

Little Angels has knitted baby woollens in our own unit in Mumbai since 2004, and almost everything on this page is in the design of a Little Angels sweater set: front-opening cardigans, a tie-knot cap and socks made to the same weight as the sweater, anti-pilling baby yarn, tag-less necks and flat seams, all meant to be worn over a cotton vest or jhabla. It is not that we set out to write a textbook. It is that the women who started this were mothers first, and mothers already knew what the textbook says: a baby cannot keep herself warm, so somebody has to do it for her.

See the sweater sets  ·  What should a newborn wear in winter? Month-by-month guide  ·  How to wash baby woollens without shrinking them

This article is general information for parents, not medical advice. If your baby feels cold on the chest, is unusually drowsy, is feeding poorly, or has a temperature of 38 °C or above, contact your paediatrician.