Photo Credit baby image by Dron from Fotolia.com. An infant up to 3 months old has a normal body temperature range of 97 to 100.4 degrees Fahrenheit, 36 to 38 degrees Celsius, according to the American Academy of Pediatrics.Temperatures both higher and lower than this range are a cause for concern.hoto Credit baby image by Dron from Fotolia.com. An infant up to 3 months old has a normal body temperature range of 97 to 100.4 degrees Fahrenheit, 36 to 38 degrees Celsius, according to the American Academy of Pediatrics. Temperatures both higher and lower than this range are a cause for concern.