If a rash has just broken out, it is probably safe to wait on calling the doctor right away. If it is not uncomfortable for your child, wait it out prior to giving the office a call.
Take a look at where the rash is and see if you can determine a reasonable cause. For example, a red and bumpy rash under the neck or armpits that breaks out after coming in from a long day or afternoon in the sun could be a heat rash will likely disappear after the heat is gone.
If the rash is on your child's legs and you have recently washed his pants in a new laundry detergent, try waiting out the rash and switching back to your previous soap.
If your baby develops a rash, it could be eczema, a skin disease that typically appears on the cheeks, forehead and scalp. As the child gets older, it can spread to the trunk, arms and legs. Using fragrance-free soap and laundry detergent along with topical ointments can keep eczema in check.