At around 3 to 4 weeks of age, some babies get tiny, firm, pus-filled pimples with a slight redness around the base on their face, and these tiny blemishes can spread to the shoulders and back. (I jokingly tell parents that infant acne usually breaks out just before relatives from out of town arrive to see the new baby.) As far as I know, no one has studied whether infants who get acne as newborns have a higher incidence of adolescent acne, but the mechanism is probably similar. Hormones present at the end of your pregnancy stimulate oil glands in baby's skin, and the oil clogs pores. No treatment is necessary because the condition disappears on its own within a few weeks.
When to Worry:
Call the doctor if the pimples are not small and hard but large and squishy, and seem to have liquid pus in them, or if there is more than a little redness around the base. This may be a sign of a bacterial infection, which would need to be treated with antibiotics.