A speciic sequence of events leads to bleeding in cases in which the placenta is located over the internal os. First, the uterine body remodels to form the lower uterine segment. With this, the internal os dilates, and some of the implanted placenta inevitably separates. Bleeding that ensues is augmented by the inherent inability of myometrial fibers in the lower uterine segment to contract and thereby constrict torn vessels. Similarly, bleeding from this lower segment implantation site also frequently continues ater placentl delivery. Last, there may be lacerations in the friable cervix and lower segment. These may be especially problematic following manual removal of a somewhat adhered placenta.
Morbidy adherent placentas are a frequent and serious complication associated with placenta previa. Described later (p. 777), this abnormally firm placental attachment derives in part from poorly developed decidua that lines the lower uterine segment. Biswas and coworkers (1999) performed placental bed biopsies in 50 women with a previa and in 50 control women. Trophoblastic giant-cell iniltration of spiral arterioles-rather than endovascular trophoblast cells-was found in half of previa specimens. In contrast, only 20 percent of biopsies from normally implanted placentas had these changes. In another study of 514 cases of previa, abnormal placental attachment was identiied in 7 percent (Frederiksen, 1999). As discussed, previa overlying a prior cesarean incision conveys a particularly high risk for morbidly adherent placenta.
Coagulation dects are rare complications of placenta previa, even when implantation site separation is extensive (Cunningham, 2015). Placental thromboplastin, which incites the intravascular coagulation seen with placental abruption, is presumed to readily escape through the cervical canal rather than be forced into the maternal circulation. he paucity of large myometrial veins in this area may also be protective.
Whenever there is uterine bleeding after midpregnancy, placenta previa or abruption are always considered. In the Canadian Perinatal Network study discussed earlier (p. 758), placenta previa accounted for 21 percent of women admitted from 22 to 28 weeks' gestation with vaginal bleeding (Sabourin, 2012). Previa should not be excluded until sonographic evaluation has clearly proved its absence. If sonography is not readily available, diagnosis by clinical examination is done using the double set-up technique because it requires that a inger be passed through the cervix and the placenta palpated. A digital examination should not be performed unless delivery is planned. A cervical digital examination is done with the woman in an operating room and with preparations or immediate cesarean delivey. Even the gentlest examination can cause torrential hemorrhage. Fortunately, double set-up examination is rarely necessary because placental location can almost always be ascertained sonographically.
Quick and accurate localization can be accomplished using standard sonographic techniques (American Institute of Ultrasound in Medicine, 2013). his is usually done with transabdominal sonography. If the placenta clearly overlies the cervix or if it lies away from the lower uterine segment, the examination has excellent sensitivity and negative-predictive value (Olive, 2006; Quant, 2014). Obese women may have limitations of visualization of the lower uterine segment. Also, a full bladder may artiicially elongate the cervix and compress the lower uterine segment to give the impression that the placenta overlies the cervix. If placental location remains in question, then transvaginal sonography is the most accurate method of assessment (Fig. 41-23). It is safe, even when there is bleeding.