Charles Ellicott Commentary Luke 2:5

"to enrol himself with Mary, who was betrothed to him, being great with child." — Luke 2:5 (ASV)

To be taxed.—Literally, to register himself.

With Mary his espoused wife.—Many of the best manuscripts omit the substantive: “with Mary who was betrothed to him.” The choice of the participle seems intended to imply the fact on which St. Matthew lays stress (Matthew 1:25). She went up with him, not necessarily because she too had to be registered at Bethlehem, but because her state, as being great with child, made her, in a special sense, dependent on Joseph’s presence and protection.