Luke 2:3-6

And everyone went to register for the census, each to his own city. So Joseph also went up from Galilee, from the city of Nazareth, to Judea, to the city of David which is called Bethlehem, because he was of the house and family of David, in order to register with Mary, who was betrothed to him, and was with child. [Matt 1:18-25] While they were there [in Bethlehem], the time came for her to give birth