Any irritation of the skin in the diaper area can be considered a diaper rash. They are extremely common in babies especially those who have loose stools or who have very wet diapers (such as infants over 9 months). Changes in a child's diet can also cause the stool to irritate a baby's skin, even if she is changed right away.
If the baby is not fussy, it is likely not a serious rash and will probably go away after a few days of using a diaper cream or ointment. If a baby has a fever, develops sores in the diaper area, has bleeding or pus from the skin, has a rash for longer than a few days, or seems very uncomfortable, it's important to have her checked out by the pediatrician.
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