And whence is this to me, that the mother of my Lord should come to me?
For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy.
And blessed is she that believed: for there shall be a performance of those things which were told her from the Lord.
The word used there is kyriou which is translated Lord. Look at the different versions of that word used. [http://www.biblestudytools.com/search/?q=Lord&s=References&rc=LEX&rc2=LEX GRK&p=1]
Kuriakos
belonging to the Lord related to the Lord Lord's2
Kurios
he to whom a person or thing belongs, about which he has power of deciding; master, lord the possessor and disposer of a thing the owner; one who has control of the person, the master in the state: the sovereign, prince, chief, the Roman emperor is a title of honour expressive of respect and reverence, with which servants greet their master this title is given to: God, the Messiah Lord of lords2, Lord's12, lords1, master38, master's3, masters8, masters'1, owner6, owners1, sir11, sirs1
Kurieuo
to be lord of, to rule, have dominion over of things and forces to exercise influence upon, to have power over Lord of1, lords1
To try to make people think that she was saying “mother of my God” is disingenuous at best. If she had truly meant God she would have used some form of Theos. I haven’t seen that verse used by the RCC for their justification but if they do its pure deceit. There is special condemnation throughout scripture for those who lead people astray with that type of deceit. I would suggest that verse not be used to lead people to a false interpretation.