Friday, December 28, 2007
*Definition: In placenta previa, the placenta is located over or very near the internal os. *Four degrees have been recognized: 1. Total placenta previa: The internal cervical os is covered completely by placenta. 2. Partial placenta previa: The internal os is partially covered by placenta. 3. Marginal placenta previa: The edge of the placenta is at the margin of the internal os. 4. Low lying placenta: The placenta is implanted in the lower uterine segment such that the placental edge actuallly does not reach the internal os but is in close proximity to it. -In vasa previa, the fetal vessels course through membranes and present at the cervical os. This is an uncommon cause of antepartum hemorrhage and is associated with a high rate of fetal death. -The degree of placenta previa will depend on the cervical dilatation at the time of examination. *Frequency -The zygote that implants very low in the uterine cavity is likely to form a placenta that lies in very close proximity to the internal cervical os. -In most cases, it does not become symptomatic until the late 2nd or 3rd trimester -It is about 1 in 200 deliveries, or 0.5 % *Etiology 1-Multiparity and advancing age increase the risk of placenta previa, the incidence of previa in women over 35 is 1 in 100 and for those over 40 it was 1 in 50. Conversely, the incidence was 1 in 300 for women aged 20 to 29 2-Prior cesarean delivery or induced abortion increases the placenta previa, it is about 3.9 % in women who had previously cesarean delivery. It is 3- 5fold increase. -The incidence increased with the number of previous cesarean sections; it was 1.9 % with two prior cesarean and 4.1 % with three or more prior cesarean deliveries. 3-Defective decidual vascularization, a possible result of inflammatory or atrophic changes, has been implicated in the development of placenta previa. 4-The relative risk of placenta previa to be increased 2-fold related to smoking. The CO hypoxemia caused compensatory placental hypertrophy. 5-A large placenta, which spreads over a larger area of the uterus, as seen with erythroblastosis and with multiple fetuses, predisposes to previa. 6-It may be associated with placenta accreta or placenta increta or percreta. Such abnormally firm attachment of the placenta might be because of poorly developed decidua in the lower uterine segment. - 5 % of women with a placenta previa had a clinically significant placenta accreta. For women with a prior cesarean section, the incidence was almost 25 %. Clinical Findings -The most characteristic event 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 has onset without warning, presenting without pain. The initial bleeding is rarely profuse. Usually it ceases spontaneously, only to recur. -In some cases, particularly those with a placenta implanted near but not over the cervical os, bleeding does not appear until the onset of labor. -The cause of hemorrhage 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 increased by the inability of the myometrial fibers of the lower uterine segment to contract and constrict the torn vessels. -Coagulation Defects, coagulopathy is rare with placenta previa, even when extensive separation from the implantation site has occurred. Diagnosis 1-The diagnosis 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 done in operating room .Today, a "double set up" examination is rarely necessary. 2-Sonography ,the simplest, most precise, and safest method of placental localization is provided by (1) Transabdominal sonography, the average accuracy is about 95 %, false positive results are often a result of bladder distention. Therefore, ultrasonic scans in apparently positive cases should be repeated after emptying the bladder. (2) The use of transvaginal ultrasonograpby improved diagnostic accuracy to visualize the internal os in all cases, in contrast to only 70 % using transabdominal equipment. 93 % positive predictive value and 98 % negative predictive value for TVS. (3) Transperineal sonograrphy allow visualization of the internal os in all cases examined. (4) Magnetic Resonance Imaging to visualize placental abnormalities including placenta previa It is unlikely that this will replace ultrasonic scanning routine evaluation in the near future. Placental "Migration" - Placentas that lie close to the internal cervical os, but not over it, during the second trimester, or even early in the third trimester; are very unlikely to persist as previa. 1-Low lying placenta not covering the internal os, previa did not persist and hemorrhage was not encountered. 2-Of those placenta covering the os at midpregnancy, about 40 % persisted as a previa. -The mechanism of placental movement is not completely understood. The term migration is clearly a misnomer -The apparent movement of the low lying placenta relative to the internal os probably results from inability to precisely define this relationship in a 3 -dimensional manner using 2 dimensional sonography in early pregnancy. This difficulty is coupled with differential growth of lower and upper myometrial segments. Management -Women with a placenta previa may be considered as follows: (1) Those in whom the fetus is preterm but there is no pressing need for delivery, (2) Those in whom the fetus is reasonably mature, (3) Those in labor, and (4) Those in whom hemorrhage is so severe as to mandate delivery despite fetal immaturity. (1) Management of the pregnancy complicated by placenta previa and a preterm fetus, but with no active bleeding, consists of hospitalization. The mother and her family must be prepared to transport her to the hospital immediately. Delivery - Cesarean section is the accepted method of delivery in all cases of placenta previa. In most cases a transverse uterine incision is made. Occasionally a vertical incision is recommended in some circumstances. -When placenta previa is complicated by degrees of placenta accreta, control bleeding from the placental bed by: 1 -Oversewing the implantation site with 0 chromic sutures. -Cho and colleagues (1991) described placing circular interrupted 0 chromic sutures around the lower segment, above and below the transverse incision. -Druzin (1989) described four cases in which the lower uterine segment was tightly packed with gauze that successfully arrested hemorrhage. The pack was removed transvaginally 12 hours later. 2 -In some cases, bilateral uterine artery ligation is helpful, 3 -In others, internal iliac artery ligation. 4- If these methods fail, hysterectomy is necessary. Prognosis -In 1927 Bill advocated adequate transfusion and cesarean section, this results in a marked reduction in maternal mortality from placenta previa -In1945, Macafee and Johnson independently suggested expectant therapy for patients remote from term, -Preterm delivery is a major cause of perinatal death. -The fetal growth retardation is increased with placenta previa.