Could be any number of things, ranging from simply how the baby is positioned in the womb (Remember, the skull is still malleable until some time after birth) to things like anencephaly. Ask your doctor during an ultrasound for more information.
It could also be a case of 'flat head syndrome'; plagiocephaly, brachycephaly or scaphocephaly. This may correct itself once the baby is born, but in more severe cases helmet treatment may be necessary.