1. Obstetrics, Normal and Problem Pregnancies;Gabbe,1986
2. Vaginal ultrasound for diagnosis of placenta previa;Farine;Am J Obstet Gynecol,1989
3. Second trimester placenta previa: an apparently normal placentation;Wexler;Obstet Gynecol,1973
4. Placenta previa: false negative diagnoses;Laing;J Clin Ultrasound,1981
5. Williams Obstetrics;Pritchard,1985