1. 1. WW Beck, Obstetrics and Gynecology . Fourth edition . Baltimore: Williams and Wilkins, 1997 : Chapter 2; 11 -13 .
2. 2. NA Beischer, EV Mackay, and P. Colditz, Obstetrics and the Newborn . Third edition . London: WB Saunders, 1997 : Chapter 10; 91 -97 .
3. 4. CRB Beckman, FW Ling, BM Barzansky, GW Bates, WNP Herbert, DW Laube, and RP. Smith, Obstetrics and Gynacology . Second edition . Baltimore: Williams and Wilkins, 1995 : Chapter 5; 74 -76 .
4. 5. ML Pernoll (Ed.). Current Obstetric and Gynecologic Diagnosis and Treatment . Seventh edition . Connecticut: Appleton and Lange, 1991 : Chapter 8; 194 -195 .