William was the son of William VIII of Aquitaine by his third wife, Hildegarde of Burgundy. His birth was a cause of great celebration at the Aquitanian court, but the Church at first considered him illegitimate because of his father's earlier divorces and his parents' consanguinity . This obliged his father to make a pilgrimage to Rome soon after his birth to seek Papal approval of his third marriage and the young William's legitimacy.