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.