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Nappy rash usually occurs when your baby's skin comes into contact with urine and faeces.


When a baby soils or wets itself, the nappy cannot always absorb the waste products. This means that your child's delicate skin will come into contact with urine and faeces. When a nappy is left on for a long time, the urine and faeces can turn into the chemical ammonia. Ammonia can irritate your baby's skin, causing it to become sore and inflamed.

Fungal infection

Nappy rash can also be caused by a fungal infection. If your baby's skin is warm and damp for long periods of time, it can cause a fungus, known as candida, to grow. Like ammonia, candida can irritate your baby's skin. Sometimes, your baby's rash starts as a reaction to the ammonia, then is further complicated by a fungal infection.

Underlying conditions

In rare cases, your baby's nappy rash may be caused by an underlying condition. Some of these conditions are listed below.


Eczema makes your baby's skin dry and sore. The sore skin may appear in other parts of the body beyond the nappy area. If your baby often has nappy rash despite regular nappy changes, and you have a family history of eczema, the nappy rash may be the first signs of eczema. For more information, see Health A-Z: atopic eczema

Seborrhoeic dermatitis

Seborrhoeic dermatitis causes red, scaly skin. It usually occurs when your baby is between two weeks and six months old. You may also notice reddened skin on your baby's scalp, ears, eyebrows, armpit and neck. Normally, seborrhoeic dermatitis lasts for a few weeks, and does not reoccur or disturb your baby.

Bacterial infection

Sometimes, your baby's nappy area can become infected with bacteria. This can cause a bright red, painful rash that will need to be treated with antibiotics.

Allergic dermatitis

Sometimes, your baby's rash may be caused by an allergic reaction to a particular substance (allergen). An allergen causes the body's immune system to react abnormally, leading to irritation and inflammation in the affected body part.

Many different types of allergen can cause your baby to have an allergic reaction, including:

  • soap
  • fragrances
  • preservatives
  • detergents
  • oils
  • powders

Sometimes, it is obvious which allergen is causing your baby's allergic dermatitis. For example, the rash may have developed after you used a new type of soap on their skin.

But in some cases, it may be harder to identify the exact cause of their reaction. If your GP cannot identify the allergen, they may refer your baby for allergy testing. This will involve testing your baby with very small amounts of various substances, to see if any of them trigger a reaction.

If your baby's nappy rash is caused by allergic dermatitis, treat the rash the same way that you would treat a mild nappy rash (see Treatment - nappy rash). If your baby has symptoms of a severe nappy rash after an allergic reaction, contact your GP.

If you know which allergen is causing your baby's nappy rash, make sure that they do not come into contact with it. This will help to prevent further allergic reactions.

For more information, see Health A-Z: contact dermatitis


Psoriasis is a very rare condition in babies, but when it occurs, it normally affects the nappy area. Psoriasis causes a rash with red scales that has a defined border. This condition most commonly starts when your child is around two months old. It usually lasts two to four months. For more information, see Health A-Z: psoriasis

Zinc deficiency

Zinc deficiency is more common in premature babies. It causes a rash, which appears around the nappy area, and around the mouth and hands.

view information about Nappy Rash on www.nhs.co.uk »

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