 The purpose of this trial is to determine whether cervical ripening with 40 mg of isosorbide mononitrate given intravaginally at 410, 412 and 414 weeks reduce cesarean section in nullipara with post term pregnancies. Treatments will be administered by midwifes in the maternity wards, women will be allowed to go back home in case of Bishop score6. Otherwise labor will be induced with oxytocin. If undelivered at 415, women with Bishop score6 will be induced with prostaglandins according to local protocols. 