Placenta Previa - A Low Lying Placenta

With placenta previa, the placenta is implanted in the lower uterine segment, where it encroaches on the internal cervical os. 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. Generally, termination of pregnancy is necessary when placenta previa is diagnosed in the presence of heavy maternal bleeding. Maternal prognosis is good if hemorrhage can be controlled; fetal prognosis depends on gestational age and amount of blood lost.

Causes
With placenta previa, the placenta may cover all (total, complete, or central), part (partial or incomplete), or a fraction (margin 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. 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.

With placenta previa, the lower segment of the uterus 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. For unknown reasons, eccentric insertion of the umbilical cord often develops. Hemorrhage occurs as the internal cervical os effaces and dilates, tearing the uterine vessels.

Signs and symptoms
Placenta previa usually produces painless third-trimester bleeding (typically the first complaint). Because of the placenta’s location, various malpresentations occur that interfere with proper descent of the fetal head. (However, the fetus remains active, 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, performed only immediately before delivery. In most cases, only the cervix is visualized.

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 a hemoglobin level and hematocrit as well as typing 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—access the degree of placenta previa and necessary fluid and blood replacement have been determined—treatment consists of careful observation, which allows the fetus more time to mature. If clinical evaluation confirms total placenta previa, the patient will likely be hospitalized because of the increased risk of hemorrhage. 

Immediate delivery by cesarean section may be necessary when the fetus is sufficiently mature or sooner if the patient experiences severe hemorrhage. 

Vaginal delivery is considered only when bleeding is minimal and the placenta previa is marginal or when labor is rapid. Because of the possibility of fetal blood loss through the placenta, a pediatric team should be on hand during the 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 also recommended.
• Prepare the patient and her family for a possible cesarean section 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.