I heard Mary’s parents are saints. If that is true, why? They weren’t the ones carrying the Lord to full term.

They might not have been the ones carrying Jesus to full term but Catholic belief is that Joachim and Anne–the traditional names given to Mary’s parents–were holy people in their right.  (They are, by the way, the patron saints of grandparents.)   We don’t know much about them, if anything at all, other than they must have been devout Jewish parents.  And holiness need not mean doing anything as extraordinary as Mary did–holiness makes its home in humanity.  Moreover, holiness in the Gospels often includes imperfections, too–think of St. Peter, who denied Jesus, and St. Thomas, who doubted him.  Anyway, when it comes to the saints, Joachim and Anne are among my favorites: simple people who probably did their simple task with great love.  And the task of raising Mary, which seemed perhaps so ordinary, produced some extraordinary results.
James Martin, SJ author of My Life with the Saints.