OP18.04: 3D ultrasound evaluation of progression angle, progression distance and midline angle changes in nulliparous women according to second stage length. - Maroni, E.; De Musso, F.; Nanni, M.; Giorgetta, F.; Youssef, A.; Bellussi, F.; Salsi, G.; Cariello, L.; Arcangeli, T.; Rizzo, N.; Pilu, G.; Ghi, T. (Ultrasound in Obstetrics & Gynecology, 2012)