Placenta previa
Placenta previa: Excerpt from Professional Guide to Diseases (Eighth Edition)
In placenta previa, the placenta is implanted in the lower uterine segment, where it encroaches on the internal cervical os. Generally, termination of pregnancy is necessary when placenta previa is diagnosed in the presence of life-threatening maternal bleeding. Maternal prognosis is good if hemorrhage can be controlled; fetal prognosis depends on gestational age and amount of blood lost. Anemia may be managed by blood transfusion to permit the pregnancy to continue in utero.
Causes and incidence
In placenta previa, the placenta may cover all (total, complete, or central), part (partial or incomplete), or a fraction (marginal or low-lying) of the internal cervical os. The degree of placenta previa depends largely on the extent of cervical dilation at the time of examination because the dilating cervix gradually uncovers the placenta. (See Three types of placenta previa.) Although the specific cause of placenta previa is unknown, factors that may affect the site of the placenta’s attachment to the uterine wall include:
❑ defective vascularization of the decidua
❑ multiple pregnancy (the placenta requires a larger surface for attachment)
❑ previous uterine surgery
❑ multiparity
❑ advanced maternal age.
In placenta previa, the uterus’ lower segment fails to provide as much nourishment as the fundus. The placenta tends to spread out, seeking the blood supply it needs, and becomes larger and thinner than normal. Eccentric insertion of the umbilical cord often develops, for unknown reasons. Hemorrhage occurs as the internal cervical os effaces and dilates, tearing the uterine vessels.
This disorder, one of the most common causes of bleeding during the second half of pregnancy, occurs in approximately 1 in 200 pregnancies, more commonly in multigravidas than in primigravidas.
Signs and symptoms
Placenta previa usually produces painless third-trimester bleeding (often the first complaint). Various malpresentations occur because of the placenta’s location and interfere with proper descent of the fetal head. (The fetus remains active, however, with good heart tones.) Complications of placenta previa include shock or maternal and fetal death.
Diagnosis
Special diagnostic measures that confirm placenta previa include:
❑ transvaginal ultrasound scanning for placental position
❑ pelvic examination (under a double setup because of the likelihood of hemorrhage), performed only immediately before delivery to confirm the diagnosis. In most cases, only the cervix is visualized.
Digital examination should be deferred in any pregnant woman in the third trimester with vaginal bleeding, until ultrasound rules out placenta previa.
Treatment
Treatment of placenta previa is designed to assess, control, and restore blood loss; to deliver a viable infant; and to prevent coagulation disorders. Immediate therapy includes starting an I.V. line using a large-bore catheter; drawing blood for hemoglobin levels and hematocrit as well as type and crossmatching; initiating external electronic fetal monitoring; monitoring maternal blood pressure, pulse rate, and respirations; and assessing the amount of vaginal bleeding.
If the fetus is premature, following determination of the degree of placenta previa and necessary fluid and blood replacement, treatment consists of careful observation to allow the fetus more time to mature. If clinical evaluation confirms complete placenta previa, the patient may be hospitalized because of the increased risk of hemorrhage. As soon as the fetus is sufficiently mature, or in case of intervening severe hemorrhage, immediate delivery by cesarean birth may be necessary. Vaginal delivery is considered only when the bleeding is minimal and the placenta previa is marginal, or when the labor is rapid. Because of the possibility of fetal blood loss through the placenta, a pediatric team should be on hand during such delivery to immediately assess and treat neonatal shock, blood loss, and hypoxia.
Complications of placenta previa necessitate appropriate and immediate intervention.
Special considerations
❑ If the patient shows active bleeding because of placenta previa, a primary nurse should be assigned for continuous monitoring of maternal blood pressure, pulse rate, respirations, central venous pressure, intake and output, amount of vaginal bleeding, and fetal heart tones. Electronic monitoring of fetal heart tones is recommended.
❑ Prepare the patient and her family for a possible cesarean birth and the birth of a premature infant. Thoroughly explain postpartum care, so the patient and her family know what measures to expect.
❑ Provide emotional support during labor. Because of the infant’s prematurity, the patient may not be given analgesics, so labor pain may be intense. Reassure her of her progress throughout labor and keep her informed of the fetus’condition. Although neonatal death is a possibility, continued monitoring and prompt management reduce this prospect.
❑ Placenta previa, especially in women who have had one or more cesarean births, is associated with placenta accreta, a dangerous condition in which the placenta grows into the myometrium.
Pictures
Book Source Details
- Book Title: Professional Guide to Diseases (Eighth Edition)
- Author(s): Springhouse
- Year of Publication: 2005
- Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.
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