Mary's conception was not purely the work of nature

When the winter of unbelief was about to end, as the author of the Canticle says (cf. Sg 2:11), and the flowers appeared on the earth to make the springtime of salvation shine forth, and when the shadows of the Law began to recede and dissipate, while the day of grace was already beginning to blow and grow more luminous, it was necessary to build and prepare the bridal chamber that could receive the Lord, so that the Bridegroom, on entering, would unite with our nature and espouse it with his goodness....

Although Anne did not give birth to the Mother of God without human intercourse (for this was only reserved for the Theotokos from the beginning of time), since her womb was sterile and unfruitful, and therefore in this respect dead, how could her womb have produced the works of the living without the presence of the divine Spirit, "who gives life to the dead and calls into existence things that are not"? (Rom 4:17).

Therefore, from a barren woman was born this entirely good daughter, so that her coming into existence was not only the work of nature, but also of the synergy of the Holy Spirit.

Thus, from the beginning, Mary was united to the Spirit, the source of life: not a single particle of her being came into existence without the participation of the Spirit [...].

If, in fact, it is incomparably extraordinary that a virginal womb should give birth, that a sterile and lifeless womb should give birth is also a singular and extraordinary thing, a work of the sole power of God.

Then, having come into existence in such a new way, this treasure and gift worthy of God, chosen before the ages and consecrated for the service of the terrible mystery of God's incarnation, the Holy Spirit was her guardian and guide, her arbiter, her honor, and in a certain way, the para-nymph who prepared her as a gracious bride for God the Father, who accepted her to become the Mother of his beloved Son.

Theophanes of Nicaea,

Discourse on the Mother of God, § 30