Luke 2:4,5 - Coke's Commentary on the Holy Bible

And Joseph also went up— Herod's order for the taxation being, as we observed on the last verse, that every one should repair to the city of his people to be enrolled, Joseph and Mary, the descendants of David, went from Nazareth, the place of their abode, to Bethlehem, the city where David and his ancestors were born: 1 Samuel 20:6; 1 Samuel 20:29. Accordingly Boaz, David's great-grandfather, calls it the city of his people; Ruth 3:11. See on Matthew 2:1. Joseph is said to be of the house and lineage of David, which Dr. Doddridge renders, of the family and household of David; supposing with Grotius, that it refers to the divisions of the tribes into families and households. Compare Numbers 1:18; Numbers 1:54. In this sense of the words, after having told us that Joseph was of the house of David, it would have been very unnecessary to add, he was also of his family; but it was not improper to say, that he was of his family and household. It may seem strange that Mary, in her condition, should have undertaken so long a journey: perhaps the order of the census required that the wife as well as the husband should be present; or, the persons to be taxed being classed in the roll according to their lineage, Mary might judge it proper on this occasion to claim her descent from David, in order to her being publicly acknowledged as one of his posterity; and the rather as she knew herself to be miraculously with child of the Messiah. However, all this was done by the divine direction; for, questionless, whatever the emperor's commands were, such a case as Mary's must have been admitted as a full excuse for her not complying with it.