They may have been biased in determining who would

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they may have been biased in determining who would have a cesarean and who would have a vaginal delivery. Two more studies have evaluated the relation between placenta–os distance and mode of delivery. In a retrospective study of 45 women with a placenta–os distance of <2 cm who were allowed to go into labor, Bronsteen et al. found that 29 (64.4%) successfully delivered vaginally [ 44 ]. When this dis- tance was <1 cm, the vaginal delivery rate was only 27.3%, whereas it was 76.5% when the placenta–os distance was 1–2 cm. Vergani and coworkers similarly found a high rate of successful vaginal delivery (69%) when the placental edge was 11–20 mm from the internal os, whereas the cesarean delivery rate was high: 75% among women who had a placental edge–os distance of 1–10 mm [ 45 ]. Summary When the placenta does not overlie the internal os and there are no contraindications to vaginal delivery, it appears that women may safely attempt a vaginal delivery if the placental edge–internal os distance in >1 cm. It must be emphasized that this assessment of placental edge–os distance can only be made accurately using TVUS. Based on the fact that the placental edge–os distance is probably the most important Caution Box Placenta previa tips 1. Most cases of placenta previa found in the second trimester will resolve prior to full term. Thus patients can be reassured in most cases. 2. Transvaginal sonography is the ideal method for diagnosing a placenta previa. Transabdominal sonography is not sufficiently accurate, and will overdiagnose placenta previa. Transvaginal sonography is safe and accurate. 3. Women with a complete placenta previa will need a cesarean delivery. Women in whom the placental edge is greater than 1 cm from the internal os may attempt for vaginal delivery, in the absence of other contraindications.
143 8 Placenta Previa and Placenta Accreta determinant of the ability to undergo a vaginal delivery successfully, Oppenheimer and Farine have proposed that the old classification of types of placenta previa be abandoned and a more contemporary classification be adopted based on the pla- centa–os relation as demonstrated on TVUS [ 3 ]. It is preferable that the women who require cesarean delivery be delivered under controlled circumstances rather than as an emergency when the patient is acutely or massively bleeding. Hence, cesarean delivery at 37–38 weeks is advisable, even without demonstrating fetal lung maturity. Placenta Accreta The term placenta accreta is used to describe collectively various degrees of abnor- mal adherence of the placenta to the myometrium. It is used when the placenta is abnormally adherent to the myometrium. Placenta increta describes a placenta that invades the myometrium. The term placenta percreta is used when the placenta invades through the myometrium into the uterine serosa and into adjacent struc- tures, such as the bladder, ureters, bowel, and omentum.

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