Joseph went from the town of Nazareth in Galilee to the town of Bethlehem
in Judea, the birthplace of King David.  Joseph went there because he was a
descendant of David.  He went to register with Mary who was promised in
marriage to him.  She was pregnant, and while they were in Bethlehem, the
time came for her to have her baby.  
(Luke 2:4-6)