Baby Rash: How to Tell What's Mild and What Needs the ER (with the Glass Tumbler Test)

Most baby rashes aren't serious — but a rash that doesn't fade when you press a glass on it is meningitis until proven otherwise. Go to the ER immediately. Press a clear glass firmly on the rash. If you can still see the spots through the glass, call 1669 (or 999/911) now. Don't wait.

Your baby has a rash — is it heat rash, eczema, or something dangerous? It's the most-asked baby question — because there are dozens of baby rashes, some clearing within an hour, others requiring an ER visit within minutes.

This article distils NHS [1], [2], AAP [3], AAD [4], and Samitivej Hospital [5] — sorting the 12 most common baby rashes and the red flags every parent has to know.

🚨 Read This First — The Glass Tumbler Test

Before sorting types, do the glass tumbler test:

1. Get a clear drinking glass (no patterns)
2. Press the side of the glass firmly on the rash until you can see skin through the glass
3. Check whether the rash fades or stays

NHS [2]: a normal rash (allergy, heat, viral) fades under glass pressure. A meningitis rash "does not fade if you press the side of a clear glass firmly against the skin" — it stays visible.

If the rash doesn't fade AND your baby has any of these → call 1669 / emergency number now:

• High fever, stiff neck
• Unusually irritable, can't be settled, high-pitched cry
• Floppy, drowsy, unresponsive
• Refusing feeds
• Bulging soft spot

NHS [2]: "Trust your instincts and do not wait for all the symptoms to appear or until a rash develops."

The glass test takes 5 seconds. Do it every time your baby gets a rash, even if you think it's just heat.

Baby Rashes Fall into 4 Groups

1. Viral rashes (usually with fever) — hand-foot-mouth, measles, chickenpox, slapped cheek, scarlet fever
2. Itchy rashes from inside — eczema (atopic dermatitis), hives (urticaria)
3. Heat / irritation rashes — heat rash, diaper rash, baby acne, cradle cap
4. Skin infections / parasites — impetigo, scabies, molluscum

Group 5 = the meningitis rash — separated out because it is an emergency.

Group 1 — Viral Rashes (with Fever)

Hand, Foot, and Mouth (HFMD)

NHS [1]: "mouth ulcers, which can be painful" + "a raised rash of spots on the hands and feet"

Common age: 6 months – 5 years. Peaks in the rainy season (May–Aug in Thailand).

Read more: Hand-foot-mouth: symptoms, care, and when to go to the ER

Measles

NHS [1]: "a spotty rash that appears on the head or neck and spreads to the rest of the body"

A child fully vaccinated with MMR almost never gets measles. An unvaccinated child catches it easily.

Measles is not benign — it can cause death or permanent brain injury. See a doctor immediately if suspected.

Chickenpox

NHS [1]: "small, itchy spots that turn into blisters and scabs"

Starts as red spots, becomes clear blisters, breaks into sores, dries to scabs — comes in waves, not all at once.

Usually self-limiting in 1–2 weeks. Don't let your baby scratch (trim nails). See a doctor if blisters become pus-filled (bacterial superinfection).

Slapped Cheek (Fifth Disease)

NHS [1]: "a rash on 1 or both cheeks plus a high temperature, runny nose, sore throat and headache"

Bright red cheeks like a slap, then a lacy rash on arms and legs. Self-limiting — but if a pregnant mother has been exposed, contact a doctor (risk to the foetus).

Scarlet Fever

NHS [1]: "small, raised bumps that feels rough, like sandpaper"

Fine red bumps that feel like sandpaper + sore throat + fever + bright red "strawberry" tongue.

Caused by Group A Strep — needs antibiotics. See a doctor any time it's suspected.

Group 2 — Itchy Rashes

Eczema / Atopic Dermatitis

NHS [1]: "itchy, dry and cracked"

Common spots: inside elbows and knees, face, cheeks, neck — dry, itchy, red, flaking; comes and goes with seasons.

AAD [4] on day-to-day care:

• Warm baths, not hot — "Bathe your child in warm — not hot — water" and "Limit your child's time in the bath to 5 or 10 minutes". Hot water + long soaks dry skin further.
• Moisturise immediately after bathing, at least twice daily — use "a thick cream or ointment"
• Petroleum jelly — "an inexpensive, fragrance-free product that works well for many children". Plain Vaseline is fine.
• Trim fingernails so scratching doesn't break the skin and cause infection
• Soft cotton clothes, washed before first wear, sensitive-skin detergent, fully rinsed

For flares: hydrocortisone cream as prescribed by a doctor or pharmacist — strength and frequency are individual, ask first; don't self-prescribe.

See a paediatric dermatologist if eczema doesn't improve within 2 weeks of basic care, or if there's pus or weeping (suspect superinfection).

Read more: Baby month 6 — skin and bathing

Hives (Urticaria)

NHS [1]: "raised, itchy patches or spots"

Raised, itchy welts of varying sizes. Often resolve in hours and reappear elsewhere.

Common triggers: food allergy (egg, peanut, cow's milk, fish), drugs, insect stings, viruses.

⚠️ Hives + swelling of face/lips/tongue + trouble breathing → anaphylaxis → call 1669 / 999 / 911 immediately.

Scabies

NHS [1]: "very itchy raised spots" + "lines with a dot at one end"

Intense itching at night, between fingers, armpits, groin — small burrows with a dot at the end (the mite).

Spreads through close contact — everyone in the household needs treatment together. See a doctor for prescription topical treatment.

Group 3 — Heat / Irritation Rashes

Heat Rash (Miliaria)

NHS [1]: "small, raised spots" + "itchy, prickly feeling"

Tiny clear or red bumps on neck, back, chest, groin — clogged sweat ducts.

In Thailand, heat rash is very common, especially March–May, and in babies who are tightly swaddled or left in cars.

Treatment: cooler clothes, air-conditioned rooms, cool sponging — usually resolves in 1–2 days.

Read more: Baby heat rash: prevention and care in Thai weather

Diaper Rash

Red rash limited to the diaper-contact area — change diapers more often, apply zinc-oxide cream.

Read more: Diaper rash: prevention and treatment

Baby Acne

Small red bumps on cheeks, nose, forehead. Starts 2–4 weeks after birth, resolves in 1–4 months.

Read more: Baby acne: why it happens, how to care

Cradle Cap

Yellow greasy scales on scalp and eyebrows — usually resolves on its own by 6–12 months.

Read more: Cradle cap: how to care

Group 4 — Skin Infections

Impetigo

Red sore with golden-yellow crust around nose, mouth, arms.

Highly contagious — see a doctor for antibiotic treatment (topical or oral).

Molluscum Contagiosum

Small skin-coloured bumps with a central dimple — spreads by contact, usually self-resolves in 6–12 months.

⚠️ Red Flags — When to Go to the ER Now

NHS [1] says call 999 (in Thailand: 1669) if:

• ❌ Rash that doesn't fade under glass pressure — "the rash looks like small bruises or bleeding under the skin and does not fade when you press a glass against it" — possible meningococcal sepsis, fatal within hours
• ❌ Stiff neck
• ❌ Difficulty breathing, wheezing, gasping

NHS [2] for babies, also watch for:

• ❌ Refusing feeds repeatedly
• ❌ Unusually irritable, high-pitched cry, can't be settled
• ❌ Floppy, drowsy, unresponsive
• ❌ Bulging soft spot

See a doctor within 2–4 hours (urgent but not immediate):

• A rash spreading rapidly
• Rash + fever > 39°C
• Rash + facial/lip swelling
• Rash with pus or weeping (suspect bacterial infection)

Routine doctor / pharmacist visit:

• Persistent rash not improving in a week
• Rash itchy enough to disrupt sleep
• Recurrent rashes in the same spot

Summary

1. Most baby rashes aren't dangerous — but always do the glass tumbler test (5 seconds)
2. Viral rashes (with fever) = HFMD, measles, chickenpox, slapped cheek, scarlet fever — sort by location and pattern
3. Itchy rashes = eczema, hives, scabies — manage skin / find trigger. Hives + breathing difficulty = emergency
4. Heat / irritation rashes = heat rash, diaper rash, baby acne, cradle cap — change environment, self-resolves
5. Skin infections = impetigo, scabies, molluscum — see a doctor
6. Emergency — call 1669 / emergency number now: rash that doesn't fade under a glass, stiff neck, breathing difficulty, refusing feeds, floppy, bulging soft spot
7. Trust your instincts — if your baby seems wrong, even without all the symptoms, see a doctor. NHS [2]: "do not wait for all the symptoms to appear"