It couldn't be any other way - the mother of Jesus had to be two things from the lineage of David and she was through Nathan and untouched / pure / clean.
Immaculate conception is the only way the spirit of God / Holy Spirit could implant the spirit of Jesus within her.
Had neither of the above taken place Jesus would not have been born in a human body, and it doesn't matter whether it was Mary or another woman, but Mary was predestined chosen to carry him to term, and raise him.