In placenta previa (Figure 26-1), the placenta is located over or very near the internal os. This condition complicates as many as 1 in 200 deliveries. Four degrees of this abnormality have been recognized and are summarized in Table 26-1. Shown in Table 26-2 are risk factors for placenta previa. Although half of women are near term when bleeding first develops, preterm delivery still poses a formidable problem for the remainder, because not all women with placenta previa and a preterm fetus can be treated expectantly. From the perspective of the mother, adequate blood transfusion and cesarean delivery have resulted in a marked reduction in mortality from placenta previa.
FIGURE 26-1 Total placenta previa showing that copious hemorrhage could be anticipated even with modest cervical dilation. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
TABLE 26-1. Classification of Placenta Previa
TABLE 26-2. Risk Factors for Placenta Previa
Another condition, termed vasa previa, is diagnosed when the fetal vessels course through membranes and are present at the cervical os. With vasa previa (Figure 26-2), there is considerable danger to the fetus, for rupture of the membranes may be accompanied by rupture of a fetal vessel, causing exsanguination. Unfortunately, the amount of fetal blood that can be shed without killing the fetus is relatively small. A quick, readily available approach for detecting fetal blood is to smear the blood on a glass slide, stain the smear with Wright stain, and examine for nucleated red cells, which normally are present in cord blood but not maternal blood.
FIGURE 26-2 Vasa previa. The placenta (bottom) and membranes have been inverted to expose the amnion. Note the large fetal vessels within the membranes (top) and their proximity to the site of rupture of the membranes. Vasa previa is diagnosed when such vessels are present at the cervical os. Note that there is a velamentous insertion of the umbilical cord.
PLACENTA ACCRETA, INCRETA, AND PERCRETA
As many as 7 percent of cases of placenta previa may be associated with placenta accreta or one of its more advanced forms, placenta increta or percreta (see Chapter 28). Such abnormally firm attachment of the placenta might be anticipated because of poorly developed decidua in the lower uterine segment associated with placenta previa.
The most characteristic event in placenta previa is painless hemorrhage, which usually does not appear until near the end of the second trimester or after. Some abortions, however, may result from such an abnormal location of the developing placenta. Frequently, bleeding from placenta previa has its onset without warning, presenting without pain in a woman who has had an uneventful prenatal course. Fortunately, the initial bleeding is rarely so profuse as to prove fatal. Usually it ceases spontaneously, only to recur. In some women, particularly those with a placenta implanted near but not over the cervical os, bleeding does not appear until the onset of labor, when it may vary from slight to profuse hemorrhage and may clinically mimic placental abruption.
The cause of spontaneous hemorrhage is related to the development of the lower uterine segment. When the placenta is located over the internal os, the formation of the lower uterine segment and the dilatation of the internal os result inevitably in tearing of placental attachments. The bleeding is augmented by the inability of the myometrial fibers of the lower uterine segment to contract and thereby constrict the torn vessels.
Hemorrhage from the placental implantation site in the lower uterine segment may continue after delivery of the placenta, because the lower uterine segment is more prone to contract poorly than the uterine body. Bleeding may also result from lacerations in the friable cervix and lower uterine segment, especially following manual removal of a somewhat adherent placenta. Coagulopathy is rare with placenta previa, even when extensive separation from the implantation site has occurred.
In women with uterine bleeding during the latter half of pregnancy, placenta previa or abruptio placentae should always be suspected. The possibility of placenta previa should not be dismissed until appropriate evaluation, including sonography, has clearly proved its absence. The diagnosis of placenta previa can seldom be established firmly by clinical examination unless a finger is passed through the cervix and the placenta is palpated. Such examination of the cervix is never permissible unless the woman is in an operating room with all the preparations for immediate cesarean delivery, because even the gentlest examination of this sort can cause torrential hemorrhage. Furthermore, such an examination should not be made unless delivery is planned, for it may cause bleeding of such a degree that immediate delivery becomes necessary even though the fetus is immature. Such a “double setup” examination is rarely necessary as placental location can almost always be obtained by sonography.
Localization by Sonography
The simplest, most precise, and safest method of placental localization is provided by transabdominal sonography, which is used to locate the placenta with considerable accuracy (Figures 26-3 and 26-4). False-positive results are often a result of bladder distention. Therefore, ultrasonic scans in apparently positive cases should be repeated after emptying the bladder. An uncommon source of error has been identification of abundant placenta implanted in the uterine fundus but failure to appreciate that the placenta was large and extended downward all the way to the internal os of the cervix.
FIGURE 26-3 Partial anterior placenta previa at 36 weeks’ gestation. The placenta margin (red arrow) extends downward toward the cervix. The internal os (yellow arrow) and cervical canal (short white arrows) are marked to show their relationship to the leading edge of the placenta. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
FIGURE 26-4 Total placenta previa. A. Transabdominal sonogram of the placenta (white arrowheads) behind the bladder covering the cervix (black arrows). B. Transvaginal sonographic image of the placenta (arrows) completely covering the cervix adjacent to the fetal head. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
The use of transvaginal ultrasonography has substantively improved diagnostic accuracy of placenta previa. Most now agree that confirmatory transvaginal imaging is indicated if the placenta is low lying or appears to be covering the cervical os by transabdominal sonography.
Placentas that lie close to the internal os, but not over it, during the second trimester, or even early in the third trimester, are very unlikely to persist as previas by term. As shown in Figure 26-5, the likelihood that placenta previa persists after being identified sonographically before 28 weeks is greater in women who have had a prior cesarean delivery.
FIGURE 26-5 Percentage of women with persistent placenta previa at delivery according to gestational age at diagnosis and with and without a prior cesarean delivery. Shown as means with error bars that represent 95 percent confidence intervals. (Asterisks note that p < 0.05 comparing women with prior cesarean delivery with multiparous women with no prior cesarean delivery.) (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. Data from Dashe JS, McIntire DD, Ramus RM, et al: Persistence of placenta previa according to gestational age at ultrasound detection. Obstet Gynecol 99:692, 2002.)
The mechanism of apparent placental movement is not completely understood. The term migration is clearly a misnomer, however, as invasion of chorionic villi into the decidua on either side of the cervical os will persist. The apparent movement of the low-lying placenta relative to the internal os probably results from the inability to precisely define this relationship in a three-dimensional manner using two-dimensional sonography in early pregnancy. This difficulty is coupled with differential growth of lower and upper myometrial segments as pregnancy progresses. Thus, those placentas that “migrate” most likely never had actual circumferential villus invasion that reached the internal os in the first place.
Management with a preterm fetus, but with no active bleeding, consists of close observation. In some cases, prolonged hospitalization may be ideal; however, discharge may be considered if the bleeding has ceased and the fetus is judged to be healthy. Importantly, the woman and her family must fully appreciate the problems of placenta previa and be prepared to transport her to the hospital immediately. Delivery is affected when bleeding due to placenta previa is encountered near term or beyond.
Cesarean delivery is necessary in practically all cases of placenta previa. In most cases, a transverse uterine incision is made. Because fetal bleeding may result from an incision into an anterior placenta, a vertical incision is sometimes recommended in these circumstances. Even when the incision extends through the placenta, however, maternal or fetal outcome is rarely compromised.
Because of the poorly contractile nature of the lower uterine segment, there may be uncontrollable hemorrhage following placental removal. When the placenta previa is implanted anteriorly in the site of a prior cesarean incision, there is increased likelihood of associated placenta accreta and risk for profound hemorrhage. When placenta previa is complicated by degrees of placenta accreta that render control of bleeding from the placental bed difficult by conservative means, other methods of hemostasis are necessary. Oversewing the implantation site with 0-chromic sutures may provide hemostasis. In some cases, bilateral uterine artery ligation is helpful, and in others, bleeding ceases with internal iliac artery ligation. Commonly, hysterectomy is required to control hemorrhage.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 35, “Obstetrical Hemorrhage.”