A red rash on a newborn’s butt could be diaper rash. If a newborn’s butt gets a red rash after using a diaper, it could be due to allergies or over wrapping that leads to lack of breathability, irritation of the skin from urine and feces, plus the fact that a newborn’s butt has very tender skin can lead to a red rash. If your newborn has a red rash, you should change the diaper in time, clean up the urine and feces, and make sure that your butt is dry and clean. If you are considering a red rash caused by allergies to the diaper, it is recommended that you use a cotton diaper instead. The newborn’s butt skin is delicate and sensitive, parents must pay attention, do a good job of local cleaning, change diapers, if serious, if necessary, should seek medical attention.