Luke 1:35-38 (ESV)
And the angel answered her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God.  And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren.  For nothing will be impossible with God."  And Mary said, "Behold, I am the servant of the Lord; let it be to me according to your word." And the angel departed from her.
Conceived by the Holy Spirit, that is what we confess. And here we have the Holy Spirit overshadowing Mary, so that she will conceive. It was a miraculous conception. It has nothing to do with the sinlessness of Mary. She was a sinner. She gets off a little easier for questioning the angel than Zechariah did, but she questioned nonetheless. It’s funny, the Gospels don’t deal with Mary too much, but what little they do they don’t show her in any light that could be considered sinless. The angel just reminds here nothing will be impossible for God.
Christians believe that. Nothing is impossible for God. He does what he wants. We can come up with catch 22 type things if we like, but it just shows our depth of ignorance and stupidity when we try talk of God building rocks to big for him to carry. That sort of questioning is infantile. Nothing is impossible, he can do what he wants. He could stop the world if he likes. He created it. He can have a virgin give birth as sure as he can create a woman from the rib of a man, or man from dust.
We are in no position to declare what is and isn’t possible. None whatsoever. Those who want to declare something impossible show the depth of their hubris. There is nothing scientific about such statements, there one has crossed over into the Metaphysical whether they want to or not. It does seem a bit odd though. We have many who will declare this or that impossible. These same people are at pains to give a scientific explanation for the beginning of the world, and life. (I find it odd that those who can’t tell us how life came to be, are adamant that there was only one life form to begin with, that somehow we have all evolved from one algae. Right.
In the end, Mary accepts the angel’s answer, and bows in submission, repentant she declares herself the servant of the Lord. And that is a wonderful thing. The Holy Spirit did his wonderful work, finally God’s post fall promise of a savior is coming to life, the fruit of her womb will be her seed, and not the seed of Adam.