Placenta Previa: Classification, Diagnosis, Risks, and Management of Pregnancy

Placenta previa is a condition in which the placenta is implanted in the lower uterine segment, which is poorly vascularized. In such cases, the placenta covers the cervical os. This condition occurs in approximately 0.3 % to 2 % of pregnancies during the third trimester. In earlier gestations, a similar condition is defined as chorion previa, the outer germinal membrane from which the placenta later forms (up to 16 weeks), but this condition is not a pathology in early pregnancy.

Etiology

The fundamental cause of the development of placenta previa has not been established. However, its development has been associated with endometrial injury caused by various etiological factors.

Therefore, the following risk factors have been identified that increase the chances of developing placenta previa:

• The mother’s age is more than 35 years old;
• Multiple births;
• History of pregnancy termination;
• Surgical interventions in the uterine cavity (curettage, etc.);
• Large uterine leiomyoma;
• Utilization of in vitro fertilization technologies;
• A history of uterine surgery;
• History of placenta previa;
• Smoking;
• Race (is a controversial risk factor, some studies show that risk is increased in Asian and African women).

Implantation requires an environment with an adequate blood supply. The trophoblast cells of the outer layer of the blastocyst develop into the placenta and fetal membranes. The trophoblast attaches to the decidua, a specialized mucosal lining of the uterus. Pathological mechanisms behind the risk factors can adversely affect placental vessels, disrupting placental blood flow and leading to placentation abnormalities.

Classification of placenta previa

Placenta previa is diagnosed when the placental edge is located less than 20 mm from the cervical os. The condition may be classified into central (complete) when the cervical canal is completely obstructed and and lateral (partial) when ⅔ of the cervical canal is blocked — 3D Models:

Complete (Central) Placenta Previa: Side View (Left) and Ventral View (Right)

Partial (Lateral) Placenta Previa

By contrast, there is also a condition known as a low-lying placenta. In this case, the edge of the placenta is 20 to 35 mm away.

Types of placenta previa

Symptom of placenta previa

Painless vaginal bleeding at any gestational age is a typical sign of placenta previa. Such bleeding may also be linked to sexual intercourse or pelvic examination; sometimes, the exact cause may remain unclear.

Diagnosis of placenta previa

• During a speculum examination, the placenta may be visualized if the cervix is dilated. Note that pelvic and speculum examinations should be performed in an operating room.
• Placenta previa may be identified in a timely manner if ultrasound is conducted during routine prenatal screening. Not only transabdominal but also transvaginal examination should be performed. Transvaginal sonography has been found to be safe and more accurate for the diagnosis of placenta previa. The condition is typically diagnosed between Weeks 28 and 32 of gestation, with precise measurement of the distance from the placental edge to the cervical canal. It should be stressed that transvaginal examination must be performed with caution, as it may provoke bleeding.

It is also important to emphasize that the majority of placentas identified as low-lying in early pregnancy are no longer classified as such by the end of gestation. The placenta itself does not move, but grows towards the increased blood supply in the uterine fundus. The change in the location of the placenta is the result of the formation of the growing lower segment of the uterus.

Abnormal placentation in a poorly vascularized lower uterine segment induces compensatory placental growth and an increase in surface area in response to reduced placental perfusion. This process leads to characteristic histopathological changes, including coagulative necrosis of chorionic villi and fibrin deposition in the intervillous space. Morphologic changes occurring in the placenta previa may play an important role in maintaining adequate perfusion, which may prevent adverse neonatal outcomes.

Differential diagnosis is typically performed to distinguish among conditions that may cause vaginal bleeding at any gestational age during pregnancy. During the 1st and 2nd trimesters, potential causes of this complication include:

• Subchorionic hematoma;
• Threatened abortion;
• Ectopic pregnancy;
• Cervical cancer.

In the third trimester, vaginal bleeding is most commonly caused by threatened preterm labor. However, in approximately 1 % of cases, it may result from premature placental abruption, and more rarely, from vasa previa.

It is important to note that women with placenta previa or a low-lying placenta are at a higher risk of preterm labor throughout their pregnancy. Moreover, pregnant women with placenta previa are more prone to preterm labor compared to those with a low-lying placenta.

Progesterone, cervical pessaries, and/or cervical cerclage may be quite effective in preventing unfavorable outcomes, but to date, there is no conclusive data regarding their safety and effectiveness in this patient population.

Placental abruption also presents with vaginal bleeding accompanied by severe pain.

Vasa previa is an abnormal attachment of the umbilical vessels to the fetal membranes. It occurs in 1 in 5,000 pregnancies. Vessel integrity may be compromised if the membranes rupture, whether spontaneously or due to medical intervention.

Complication of placenta previa

Placental invasion (placenta accreta spectrum) is a complication of placenta previa and should be ruled out during ultrasound examination. This condition requires in most cases a hysterectomy due to massive bleeding, but it is possible to save the uterus with routine high-tech surgery, or a technique of delayed placenta separation can be applied until the placental bed is devascularized so that the remaining placental tissue can be more safely removed. MRI is indicated if the deviation is suspected.

Radionuclide imaging and arteriography may also yield valuable retrospective data.

Treatment

• Tocolytic therapy may be considered in cases of mild bleeding occurring prior to Week 36 of pregnancy. Special medications are also administered to prevent fetal respiratory distress and to provide neuroprotective support.
• Hemostatic therapy: To halt bleeding, hemostatic agents, blood products, plasma substitutes, and fibrinolysis inhibitors are used.
• Compliance: Patients are advised to maintain bed rest, limit physical activity, and abstain from sexual intercourse.
• Anemia management: Recurrent bleeding increases the risk of anemia, requiring medical therapy with iron supplements.
• Transfusion availability: Inpatient care requires immediate access to compatible blood and blood components

Childbirth: peculiarities of cesarean section surgery

The primary delivery mode is an elective cesarean section at Week 37 of pregnancy. However, some patients may experience complications necessitating an emergency cesarean section at an earlier gestational age. This is usually caused by massive bleeding of more than 250 ml.

A vertical skin incision ensures optimal surgical access during a cesarean section. If the placenta is located within and occupies the lower uterine segment, or if the lower uterine segment is underdeveloped, a vertical uterine incision is required. After delivery of the fetus, the placenta should be separated spontaneously; manual separation of the placenta is prohibited. Women should be informed about possible uterine artery embolization or hysterectomy. Neuroaxonal analgesia is recommended, and general anesthesia is used only in emergency cases.

Natural childbirth

Patients diagnosed with a low placenta may qualify for physiologic management of labor, provided that the pregnancy is premature and amniotomy is performed early, but they have an increased risk of bleeding in the postpartum period, and delivery may end with surgery. It is worth noting that there are no data on the benefits of methotrexate.

Prognosis for the fetus

Neonatal mortality and morbidity rates are 3–4 times higher in cases of placenta previa, primarily due to premature birth.

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Prognosis for the mother

If the placenta is located on the anterior wall of the uterus, there is an increased likelihood of massive blood loss, which increases the risks of hysterectomy. This patient group is at elevated risk for the following complications:

• Blood transfusions;
• Damage to adjacent organs;
• Septicemia;
• Providing resuscitative measures;
• Development of placenta previa in the next pregnancy;
• Death.