Based on the text of the Magnificat, it sure seems like Mary knew, which would render the song 'Mary Did You Know' moot.
In addition to that, the lyrics of 'Mary Did You Know' glosses over the subversive social message Mary sings of in the Magnificat.
As Nadia Bolz Weber writes in Preaching The Whole Magnificat:
"Mary's song isn’t a docile picture of obedience singing about how great it is to be pregnant. Mary is singing of nothing less than complete overturning of the social and economic order.
You see, there’s a reason why the Magnificat is said to have terrified the Russian Czars. Because, the message is that if you find yourself rich and powerful then … watch out! This young little Jewish girl is not singing about a whole lot of good news for you."
