Blessed is she who believed, for there will be a fulfillment of the things which have been spoken to her from the Lord!"
The fact of this response from Mary having been written in poetic form is no evidence whatever that Luke was copying some document in this section. The Psalms of David are also poetry; and Mary the descendent of David proved in these lines that she was indeed a worthy member of the house of David.  Only a male chauvinist could deny that this highly favored daughter of David's line could have composed such a beautiful poem, relating it to Luke in her own words.