Diaper rash is usually caused by irritation of a baby's skin that is exposed to urine, bowel movements, diaper irritants and infections.
Diaper rash will usually increase between seven and nine months when a more varied diet can cause more irritating stools. (This is the period when your baby is trying different foods.)
Your baby may also develop a diaper rash after taking antibiotics. This kind of diaper rash may be caused by a yeast infection and will not get better without medicated ointment your child's doctor can order.
Rarely diaper irritants can cause a rash that may heal if a different brand of diaper is used.