 She was the eldest daughter of Simon of Dammartin, Count of Ponthieu (1180- 21 September 1239), and his wife Marie of Ponthieu, Countess of Montreuil (17 April 1199 – 1251).Her paternal grandparents were Alberic III, Count of Dammartin, and Mahaut de Clermont, daughter of Renaud de Clermont, Count of Clermont-en-Beauvaisis, and Clémence de Bar.Her maternal grandparents were William IV of Ponthieu and Alys, Countess of the Vexin, daughter of Louis VII of France and Constance of Castile.