Accuracy depends on the sonographic technique used. In a comprehensive study, the internal os was visualized in all cases with transvaginal sonography but in only 30 percent with transabdominal sonography (Farine, 1988). As discussed, according to the Fetal Imaging Workshop, if the placental edge is <2 cm from the internal os, but not covering it, the placenta is considered low lying (Reddy, 2014). In the absence of any other indication, sonography need not be frequently repeated simply to document placental position. At Parkland, women with a placenta previa identiied at 18 to 22 weeks' gestation with a prior cesarean delivery are evaluated again at 28 weeks and those without at 32 weeks. Restriction of activity is not necessary unless a previa persists beyond 28 weeks or if clinical findings such as bleeding or contractions develop before this time. At 32 weeks' gestation, if the placental edge is still <2 cm from the os, then transvaginal sonography is repeated at 36 weeks.
Using MR imaging, several investigators have reported excellent results in visualizing placental abnormalities. hat said, it is unlikely that this technique will replace sonography for routine evaluation anytime soon. However, MR imaging has proved useful for evaluation of morbidly adherent placenta (p. 780).
Women with a placenta previa are managed based on their individual clinical circumstances. Three prominent factors include fetal age and maturity, labor, and bleeding severity. In one study of 214 women with a previa, 43 percent had an emergency delivery, and half of these were preterm (Ruiter, 2015). But, if the fetus is immature and active bleeding subsides, close observation in an obstetrical unit is indicated. Data are sparse regarding tocolytic administration for uterine contractions. Although robust randomized trials are lacking, Bose and colleagues (201r1) recommend that if tocolytics are given, they be limited to 48 hours of administration. We categorically recommend against their use in this setting.
Ater bleeding has ceased for approximately 2 days and the fetus is judged to be healthy, a woman can usually be discharged home with instructions for "pelvic rest." Importantly, the woman and her family must ully appreciate the possibility of recurrent bleeding and be prepared for immediate transport back to the hospital. In other cases, prolonged hospitalization may be ideal.
FIGURE 41 -23 Placenta previa. A. In this transvaginal image at 34 weeks' gestation, the anterior placenta completely covers the internal cervical os outlined by arrows. B.This transvaginal image at 34 weeks' gestation depicts a posterior placenta (arrow) that just reaches the level of the internal cervical os. (Reproduced with permission from Cunningham FG: Placenta previa and morbidly adherent placenta. In Yeomans ER, Hoffman BL, Gilstrap LC III, et al (eds): Cunningham and Gilstrap's Operative Obstetrics, 3rd edition. New York, McGraw-Hili Education,o201o7b.) he frequency of emergency delivery in women with pla centa previa ranges from 25 to 40 percent (Gibbins, 2018;
Kassir, 2017). But, in properly selected patients, long-term inpatient care does not appear to add beneits compared with outpatient management (Neilson, 2003). In one random ized study of 53 women who had a bleeding previa at 24 to 36 weeks' gestation, maternal or fetal morbidity rates did not difer between management method (Wing, 1996). Of all study women, 60 percent had recurrent bleeding, and half eventually required expeditious cesarean delivery.