Why are small bumps spreading across my baby's body?

Why spreading looks frightening

A rash that stays put is easy to live with. One that was on her thighs yesterday and is creeping towards her stomach today feels like it is going somewhere, and that is the thing that gets you looking at your phone at two in the morning.

Most of the time the spread is the rash doing its normal thing. Heat rash appears where skin is warm and covered and then shows up in the next warm covered place. A mild irritation follows wherever the thing irritating it has been.

Photographs and the glass test

This was the single most practical habit we picked up. Once a day, same light, same bit of skin, in a photo.

It does two things. It tells you whether the rash is genuinely spreading or whether you have simply been looking at it more closely each time. And it gives a doctor something real to look at, which matters because rashes have a habit of fading on the way to an appointment and reappearing in the car home.

Press a clear glass or tumbler firmly onto the rash and look through it. Most rashes blanch — the colour fades under pressure and comes back when you lift the glass. A rash that stays visible does not blanch, and that needs urgent medical help.

It is worth doing this once on an ordinary harmless rash, just so you know what blanching looks like when you're calm. Learning it for the first time at 3am on a baby with a temperature is not when you want to be squinting at a glass and wondering what you're looking for.

Change one thing at a time

Our instinct when a rash appeared was to change everything at once — new detergent, different soap, a cream, fewer clothes. Then it got better and we had no idea why, and when something similar appeared a month later we were back to nothing.

So: cool her down first, because heat is the most common cause and costs nothing to remove. Plain water and pat dry. And then genuinely change nothing else for a few days.