Baby acne looks like small whiteheads. If your baby's rash is red, scaly or clusters in patches, it is most likely eczema which is an autoimmune condition of the skin and can be related to alleriges. In my children's cases it is allergy related.
ETA: Baby acne can show up right after birth, but more commonly within the first few weeks. It can last a couple of weeks to a couple of months. It usually appears on the cheeks and sometimes on the forehead, chin, and even the back. Unlike eczema, baby acne is not related to allergies. It is a reaction to residual maternal hormones.
Eczema can appear anywhere including the diaper area. It can come and go (flaring up when there is allergen exposure). If it gets bad, it can ooze and scab.
Di Linh, mama to DS1 (7), DD(4), DS2 (b 12/01/09)