A.T. Robertson Commentary Luke 2:5

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

To enrol himself with Mary (απογραψασθα συν Μαριαμ). Direct middle. "With Mary" is naturally taken with the infinitive as here. If so, that means that Mary's family register was in Bethlehem also and that she also belonged to the house of David. It is possible to connect "with Mary" far back with "went up" (ανεβη) in verse 4, but it is unnatural to do so. There is no real reason for doubting that Mary herself was a descendant of David and that is the obvious way to understand Luke's genealogy of Jesus in Lu 3:23-38). The Syriac Sinaitic expressly says that both Joseph and Mary were of the house and city of David.

Betrothed (εμνηστευμενην). Same verb as in 1:27, but here it really means "married" or "espoused" as Mt 1:24f. shows. Otherwise she could not have travelled with Joseph.

Great with child (ενκυω). Only here in N.T. Common Greek word.