When I was visiting my sister in Colorado about eleven years ago, I was paging through a book she had been using to find some medieval Christmas carols for her madrigal-esque choral group to sing, and I ran across a fascinating find.  The Book is The New Oxford Book of Carols (Oxford: Oxford University Press, 1992), in which there are several hymns under the index entry “immaculate conception” (pages 440-455) which piqued my interest.  The Roman Catholic Church did not dogmatize the notion that the Blessed Virgin was herself immaculately conceived until 1854, alleging that it had always been popularly believed, but never codified as a necessary tenet of faith until then.  They are careful, moreover, to correct what they see as Protestant misconceptions about the meaning of the term “immaculate conception,” refusing to allow it as a reference to Jesus’ sinlessness from His moment of conception (something which, of course, neither side disputes). 

But when I looked over the hymns listed under this category, I found neither explanation satisfactory.  Every one of these medieval hymns, to my surprise, was what is sometimes called a “doubting Joseph” hymn, each containing a meditation on the angel’s words to Joseph to allay his fears about taking Mary to be his wife when he learned that she was expecting a Child.  That is to say, the term “immaculate conception” was understood—at least in this connection—as a reference to the Blessed Virgin’s immaculate behavior with regard to the conception of Jesus in her womb. 

An example is the hymn "When righteous Joseph wedded was," which contains in the fourth verse the following:

"Then Joseph he, to shun the shame,

Thought her for to forsake;

But then God's angel in a dream

His mind did undertake:

'Fear not, just Joseph;

This thy wife

Is still a spotless maid;

And no consent of sin,' said he,

'Against her can be laid."

The term “spotless” is what places this hymn in the category “immaculate conception” and refers specifically to her maiden virginity.  A paraphrase might run, “Do not be afraid to take Mary as your wife; that which is conceived in her is immaculately conceived.  She is not guilty of fornication as you suppose.”  

More research is needed, but this seems to me to be a possible origin for something which soon developed in an unfortunate way.   In addition, this helpful theme—the “doubting Joseph” theme—is evidently not so much a part of the Christmas carols we still sing today as it once was, and that’s a pity, for the angelic message to the doubting Joseph has many salutary angles which could surely be used for edification.