How did Luther's Mariology evolve over time?

Ordinarily, I would go to Wikipedia for this sort of question, but unfortunately the information therein is paltry. It is clear that Luther consistently believed in Mary as *Theotokos*, as this continues to be the normal belief of all branches of Protestantism till the modern day. This answer on another question indicates, but without adequate citations, that Luther's views on the Immaculate Conception changed over time. I have seen this from other sources as well, but nothing very clear. **I am looking for an overview of how his views on Mary, especially the sinlessness and/or immaculate conception, may have changed over the course of his career.** *Note: I have researched this question a bit myself. I have had a lot of difficulty sorting through polemics (from both sides) to find accurate information. Some verified references to primary sources would be most helpful.*