You notice it one evening during a nappy change: the back of your baby's head looks a little flatter on one side. Then you start seeing it every time, and the worry sets in.

Here is what actually causes it, what you can do at home, and the few signs that are worth raising with your paediatrician.

Why flat spots happen

A newborn's skull is soft and made of separate plates that have not yet fused. That is what allows the head to pass through the birth canal, and it is also why steady pressure on one spot can flatten it slightly.

Flat spots became more common after the recommendation that babies sleep on their backs. That recommendation is right and should not change — back sleeping is the safest sleep position and it saves lives. The trade-off is simply that babies now spend more of the day on the back of the head, so the time they spend off it matters more.

Most flattening is mild, and most of it resolves on its own once the baby starts sitting, rolling and crawling.

What you can do at home

1. Change the direction your baby lies. Babies tend to turn towards the interesting side of the room — the door, the window, you. If the cot always faces the same way, the head always turns the same way. Turn the baby around so the head end and foot end swap every few days.

2. Alternate arms when feeding. Whether you breastfeed or bottle-feed, switching sides balances which side of the head rests against you.

3. Give more awake time off the back of the head. Time on the tummy, upright on your chest, or carried in arms all count. This is the single biggest lever you have.

4. Move what your baby looks at. A baby will keep the head turned towards whatever is worth looking at. Place a high-contrast card on the side the baby turns to less, and the head follows.

Making tummy time work

Everyone recommends tummy time, and most babies protest at first. Short and often beats long and once: one or two minutes a few times a day for a newborn, building to roughly twenty to thirty minutes a day in total by three to four months.

Two things make a real difference to how long a baby tolerates it.

The first is something worth looking at. In the first months, babies see strong contrast far better than colour, so a black-and-white card in front of them holds attention far longer than a colourful toy.

The second is something that reacts. A water play mat responds to every touch — the sea creatures move, so the baby keeps pressing, reaching and lifting the head. That is exactly the movement that takes pressure off the back of the skull.

When to mention it to your paediatrician

Raise it at the next check-up if your baby always turns the head to the same side and resists turning the other way, if the flattening looks more pronounced rather than less over several weeks, if the ears or face look noticeably uneven, or if your baby cannot lift the head at all by around three to four months.

A persistent preference for one side can indicate tightness in the neck muscles, which is common and usually responds well to simple physiotherapy — especially when it is picked up early. This article is general information, not a diagnosis; your paediatrician can tell you what applies to your baby.

The short version

Keep putting your baby to sleep on the back. Change the direction of the head during the day, alternate feeding sides, and build up awake time off the back of the head. Most flat spots improve on their own — and the habits that prevent them are the same ones that build the strength your baby needs for rolling, sitting and crawling.