Writing on the Jan. 1 feast of Mary, Mother of God, the cardinal said having a priest-son requires a new form of motherhood, one that involves a "discreet, but very effective and invaluably precious accompaniment in prayer."
Cardinal Piacenza's letter was posted, in Italian, on the website of the clergy congregation.
When a man becomes a priest, he said, everyone in his family is touched and is called to a deeper conversion, but "unique and special are the spiritual consolations that come from having carried in your womb one who becomes a priest in Christ."
Obviously, he said, seminary studies and priestly ministry often take a man further from home and from regular family life, but the physical separation is replaced by a closer spiritual bond, he said.
"The experience of the church teaches that the mother 'receives' her priest-son in a completely new and unexpected way, so much so that by the will of God she is called to recognize in the fruit of her womb a 'father,' who is called to generate a multitude of brothers and sisters and accompany them to eternal life," the cardinal wrote.
While "every mother of a priest is mysteriously a 'daughter of her son,'" Cardinal Piacenza said, she also is called to continue offering him her maternal support, particularly through her prayers.
"Such a work of authentic support, always necessary in the life of the church, seems even more urgent today -- especially in the secularized West, which is awaiting and asking for a new and radical proclamation of Christ," he said. "The mothers of priests and seminarians truly represent an army that raises prayers and offerings to heaven from earth and, with even greater numbers, intercedes from heaven so that grace is poured out on the lives of holy pastors."