Premature infants can have more molding of the head than full term infants. Usually, the head of extremely premature infants tends to be more "dolichocephalic", which means longer front-to-back than it is wide.
An infant's skull still grows because the growth plates are not fully solidified with each other: this allows them to grow larger from their edges. This also means that the infant skull is easier to deform, either from pressure, or from having the edges of the plates grow together earlier than normal.
You should discuss with your pediatrician whether the shape of your baby's skull is within the normal changes of newborn infants. Most infants will have normal skull growth as they become more active and turn their heads more, allowing the skull to receive pressure from many directions. Some infants will require a "helmet" to will help their skulls to normalize their growth. Your pediatrician will work with a craniofacial specialist if this is the case.