Perhaps I misread your post, but doesn't the "Immaculate Conception" refer to the conception of Mary and not the conception of Jesus?
Maybe I don't understand the term "immaculate conception". My church doesn't use the term, thus the quotes, so I have to go by my understanding. I understand it to mean Mary's conception of Jesus. Mary being a virgin and thus immaculate.
I have heard that some churches also believe that Mary was born from a virgin mother so that Mary herself does not fall under the curse of original sin and that is why she is considered immaculate, and thus her conception is immaculate. Not of original sin, and not committing original sin.