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

Placenta previa is a condition in which the placenta implants in the poorly supplied lower segment of the uterus, covering the internal pharynx of the cervix. This condition develops in 0.3% to 2% of pregnancies in 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, there is an association between the formation of placenta previa and endometrial damage by various etiologic 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;
• Placenta previa history;
• 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 decidual membrane of the uterus. The mechanism of action of pathologies constituting risk factors has a negative effect on the formation of placental vessels, it disrupts placental blood flow, which leads to the formation of placentation abnormalities.

Classification of placenta previa

According to the classification, placenta previa is diagnosed when the edge of the placenta is less than 20 mm from the internal yawn. In turn, placenta previa is divided into central (or complete) with overlap of the entire cervical canal

and lateral (or partial) – in this case, the axis of the cervical canal is closed only by ⅔.

There is also a condition called low placenta. In this case, the edge of the placenta is 20 to 35 mm away.

Types of placenta previa

Symptom of placenta previa

Vaginal bleeding at any gestational age with no pain is a typical manifestation of placenta previa. Also bleeding with no pain syndrome can be after sexual intercourse, vaginal examination, and sometimes the cause may not be present.

Diagnosis of placenta previa

• The placenta can be visualized during the mirror examination in case of a dilated cervix. It is important to say that vaginal examination and mirror examination are performed only in the operating room.
• Sonography in the screening period of pregnancy provides timely identification of placenta previa. 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 diagnosis is established from the twenty-eighth to thirty-second week of pregnancy, and the distance from the border of the placenta to the cervical canal is accurately measured. However, care should be taken when performing a transvaginal scan, as it can cause bleeding.

It should also be clarified that most of the placentas identified as low-lying in early pregnancy are not visualized by the end of pregnancy. 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 the poorly vascularized lower uterine segment causes compensatory placental growth and increased surface area in response to decreased placental perfusion, causing corresponding histopathologic changes in the form of coagulative necrosis of the 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.

The differential diagnosis is usually between conditions that cause vaginal bleeding during pregnancy at any term. In the 1st and 2nd trimesters , this complication can cause:

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

In the third trimester, vaginal bleeding is usually caused by the threat of preterm labor, but in 1% of cases it is caused by premature detachment of the normally located placenta and rarely by umbilical cord prolapse. It is worth noting that women with placenta previa and low placenta have an increased risk of preterm labor throughout pregnancy. Pregnant women with placenta previa have a higher risk of preterm labor, relative to a low placenta. Progesterone administration, cervical pessary and/or serclage may be potentially effective prophylactic interventions, but systematic data on the safety and efficacy of these methods are lacking.

Premature detachment of the normally located placenta is also manifested by vaginal bleeding, but with accompanying severe pain syndrome.

Umbilical vascular malposition is an abnormal attachment of the umbilical cord vessels to the membranes. It occurs in 1 in 5,000 pregnancies. Vascular integrity may be compromised by rupture of the membranes of spontaneous or induced etiology.

Complication of placenta previa

True placenta ingrowth is a complication of placenta previa and should be excluded during ultrasound. 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. If true placental ingrowth is suspected, MRI should be performed.

Diagnostic methods that are of historical value include radioisotope imaging and arteriography.

Treatment

• Tocolytic therapy may be considered in cases of bleeding at less than 36 weeks’ gestation and minor bleeding, and drugs are used to prevent fetal respiratory distress syndrome and neuroprotective protection.
• Hemostatics, blood and plasma replacement drugs, and fibrinolysis inhibitors are used to stop bleeding.
• The patient is advised to follow bed rest, reduce activity and avoid sexual intercourse.
• With recurrent bleeding, there is a risk of anemia, which also requires medication in the form of iron preparations.
• And also when a patient with such a diagnosis is hospitalized, the appropriate blood type and its components should be available for quick access, if necessary.

Childbirth: peculiarities of cesarean section surgery

Planned caesarean section at 37 weeks remains the main method of delivery. However, some patients may have complications that require an emergency caesarean section at an earlier gestational age. This is usually caused by massive bleeding of more than 250 ml. A vertical skin incision is recommended during cesarean section for optimal access. In cases where the placenta is located and occupies the lower uterine segment or if the lower uterine segment is not sufficiently formed, 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

The magnitude of neonatal mortality and morbidity in the development of placenta previa increased 3-4 times, it is associated, as a rule, with premature labor.

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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 group of patients is at increased risk of complications such as:

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