Mary Could Not Have Conceived Jesus Accept She First Agree

Your free will is not an illusion.  Though you are purposed by design, and like Jeremiah, were called from the womb, you can live your entire life outside of God's presence and purpose.  Indeed, the unbelieving world is the ultimate proof of this.

So when considering the divine conception and subsequent virgin birth, you should presume Mary had no less freedom to disobey God's will.  In fact, she demonstrated her agreement and compliance in several ways.

First and definitely foremost is that she believed.  As Jesus said in Mark 16:17, the signs follow the believing, not the other way around.  Mary had first to know within her heart that the virgin birth as prophecied in Isaiah 7:14 was true.  From God's perspective, knowing this truth resided in Mary's heart was His first indication she was the right choice to bare His son.

Secondly, she was virgin.  Not to imply she maintained her virtue waiting to be God's pick as the mother of Christ; she was a virgin as a matter of law.  But again from God's perspective, Mary's virginity speaks to the purity of her womb and, perhaps more importantly, Jesus born from a virgin could not be construed as someone else's son.

Also not to be overlooked was Mary's lifestyle of holiness.  Living your life set apart unto God and His principles obviously makes you very trustworthy to Him.  It could be said that Mary did well to honor the authorities in her life, regularly attended the synagogue, studied and obeyed the scripture, etc.  I strictly speculate here on Mary's life.  I've read nothing that indicates Mary's life reflected such behavior.  Still, it's not far-fetched to believe.  She did get become Jesus' mother afterall. 

Finally and very intricate to the birth of God's will into the earth in any form, Mary verbalized her compliance.   An angel brought the message of God's purpose for Mary in that moment, but it was only when she responded saying, "Let it be done to me according to what you have said," (Luke 1:38, Amp) did the angel depart from her.

Mary had free will; so do you.  She may have been purposed as the mother of Christ, but God's will can be accomplished by anyone.  Obey the voice of the Holy Spirit in your heart and fulfill God's purpose for your life and make everyday the birthdate of God's will in the earth.