The participle phrase "she being pregnant" should be understood in the causal sense: "since she was pregnant." For this very reason Joseph took her along.But not because her pregnancy necessitated her enrollment because of the expected child ; or because Joseph and she wanted the child to be born in David's city and not Nazreth. The matter is more intimate . Joseph took Mary away from the Nazareht in order to shield her against slanderous tongues, and in order to give her all the protection and help she needed in her condition, and not to leave her in the care of others. Mary herself must also have greatly desired to leave Nazareth with Joseph. (Lenski p122) With this reading it is not innevitable that Mary accompany Joseph but rather is dependant on Joseph's concern and protective nature. This also overturns the sermons which state that God arranged an empire wide census to ensure Jesus was born in Bethlehem. (The details of the census are unclear and leaving Mary in her last trimester with other women back in Nazareth would have been "normal" practice) Lenski is a Greek scholar and I find his observation interesting and intriguing. What do others make of his reading.