A woman does not become a mother until she gives birth. At the time of Jesus' birth, Joseph and Mary were married.
While it's true that the story of her virgin impregnation (Immaculate Conception) is a core belief of the Christian faith, and is one of the means by which Christians assert the divinity of Jesus (Luke 1:24-37), the biblical author made sure to point out that Mary was married when Jesus was born.
Luke 1: 41 When Elizabeth heard Mary's greeting, the baby leaped in her womb, and Elizabeth was filled with the Holy Spirit. 42 In a loud voice she exclaimed: "Blessed are you among women, and blessed is the child you will bear! 43 But why am I so favored, that the mother of my Lord should come to me? 44 As soon as the sound of your greeting reached my ears, the baby in my womb leaped for joy. 45 Blessed is she who has believed that what the Lord has said to her will be accomplished!" 46 And Mary said: "My soul glorifies the Lord 47 and my spirit rejoices in God my Savior,
Hogwash. I am the mother of ten children. Eight of them were born.
At the time of Jesus' birth, Joseph and Mary were married.
St. Luke says that Joseph travelled to Bethlehem with "Mary, his betrothed, who was with child. And while they were there, the time came for her to be delivered, and she gave birth to her firstborn son, and wrapped Him in swaddling clothes, and laid Him in a manger, because there was no place for them in the inn."
There's no mention of a wedding ceremony between "his betrothed" and "she gave birth."
While it's true that the story of her virgin impregnation (Immaculate Conception) is a core belief of the Christian faith ...
The Immaculate Conception refers to Mary's being conceived without the effects of Original Sin, by her parents by natural means. It does not refer to the virginal conception or Virgin Birth of Jesus Christ. Many Christians do not believe in the Immaculate Conception, but the conception of Christ by the Holy Spirit, without the agency of a human man, is a core belief.