What does the presence of black and blue macules on a four-week-old's buttocks suggest?

The presence of black and blue macules, particularly on the buttocks of a four-week-old infant, is indicative of Mongolian spots. These flat, blue-gray birthmarks are commonly found on the lower back and buttocks of infants of certain ethnic backgrounds, such as those of Asian, African, and Hispanic descent. They are caused by pigmented cells (melanocytes) that have not yet migrated to the upper layers of the skin during fetal development.

Mongolian spots are generally harmless and often fade significantly or disappear entirely during early childhood. Their appearance during infancy is quite typical and does not raise concerns about injury or trauma, which differentiates them from other possibilities like bruising, which would suggest physical trauma, or conditions like erythema toxicum or pustular melanosis, which present with different characteristics and are not primarily associated with blue macules on the buttocks.