Pathogenesis: why does bleeding occur?
The morphological basis of the pathology is the development of chorion frondosum in the area of the decidua capsularis.
During pregnancy, bleeding starts during the period of active formation of the lower uterine segment. The mechanism is as follows:
- The muscle fibers of the lower uterine segments contract and shift.
- The placenta has no muscular elements and cannot contract.
- Because of this disparity, tissue shifting occurs: the chorionic villi detach from the uterine wall.
- The vessels of the placental bed are exposed, and loss of maternal blood begins.
Bleeding stops only when the uterus relaxes and blood clots form in the exposed vessels. As soon as contractions resume, a relapse occurs.
During labor, detachment is triggered by the tension of the fetal membranes, which pull on the edge of the inelastic placenta. With complete previa, spontaneous delivery is physically impossible: as the cervix effaces, the placenta will continuously detach and bleeding will intensify.
Clinical presentation
Previa can be suspected by indirect signs: the presenting part of the fetus is high, and breech, oblique, or transverse lie are common.
Characteristics of bleeding:
- Occurs suddenly, most often in the late 2nd to 3rd trimester or with the onset of contractions.
- The blood is bright red, flowing externally (internal hematomas do not form).
- Pain is completely absent.
- The uterus remains painless upon palpation, and its tone is not increased.
The intensity of blood loss directly depends on the diameter and number of damaged vessels. Even scanty but regularly recurring discharge inevitably leads to maternal anemia, which entails acute fetal hypoxia.
Diagnosis and management of pregnancy
The gold standard for diagnosis is ultrasound. Vaginal examination is strictly not recommended as it can provoke fatal detachment. It is performed only in extreme cases (if ultrasound is unavailable), acting as gently as possible and strictly within a prepared operating room ready for emergency cesarean section (CS).
Management in the 2nd trimester:
- No bleeding: outpatient observation, sexual and physical rest, ultrasound follow-up every 3–4 weeks (assessment of placental migration).
- Presence of discharge: mandatory hospitalization. In case of massive blood loss — minor CS. With scanty discharge — conservative tocolytic therapy, monitoring of hemostasis, and treatment of anemia.
Management in the 3rd trimester: Expectant management is applied until 38 weeks. It includes strict bed rest, laboratory monitoring (CBC, coagulogram) and fetal condition assessment (NST, dopplerometry). Prior to 34 weeks, prevention of fetal respiratory distress syndrome (RDS) with glucocorticoids (e.g., dexamethasone or betamethasone 12 mg twice 12 hours apart) is mandatory.
Routing: If the patient lives far from a Level III hospital, antenatal hospitalization is indicated as early as 30–32 weeks. Scheduled hospitalization for all others is at 34–35 weeks.
Algorithm for hemorrhage and delivery
If a pregnant woman is admitted with bleeding, the volume of blood loss and vital functions are evaluated first. Infusion of crystalloids and colloids is prescribed, blood tests are taken (CBC, hemostasiogram, acid-base balance, lactate), and cross-matching for donor red blood cell mass is performed.
Choice of delivery method:
- Emergency CS: performed in case of heavy bleeding at any gestational age.
- Elective CS: performed at 38 weeks if there has been no bleeding.
- Vaginal delivery: permitted only when three conditions coincide: partial previa, spontaneous onset of contractions, and complete absence of blood discharge.
Features of vaginal delivery: Early amniotomy in the operating room is mandatory. Rupture of the membranes relieves membrane tension, allowing the placenta to shift along with the lower segment. The descending fetal head mechanically presses (tamponades) the detached area against the uterine wall, stopping the bleeding. If contractions are weak after amniotomy or the head is high, intravenous oxytocin is indicated (5 IU per 500 ml normal saline).
Briefly: answers to topical questions
What types and classifications of placental previa exist?
There are clinical and ultrasound classifications of placental previa.
Clinical variants:
- Lateral previa (placenta praevia lateralis) — part of the placenta presents within the internal os, with fetal membranes nearby.
- Marginal previa (placenta praevia marginalis) — the lower edge of the placenta is located strictly at the margin of the internal os.
- Complete (central) previa (placenta praevia centralis) — the placenta completely covers the area of the internal os.
Ultrasound classification (grades):
- Grade I — the edge does not reach the internal os.
- Grade II — the edge reaches the internal os but does not cover it.
- Grade III — the edge asymmetrically covers the internal os.
- Grade IV — the placenta symmetrically covers the internal os.
What risk factors and etiological causes contribute to placental previa?
Placental previa is promoted by uterine and fetal factors.
- Uterine factor — caused by dystrophic changes in the endometrium that impair conditions for normal placentation. Risk factors include high parity, uterine scars after surgical interventions (cesarean section, myomectomy), uterine curettage, chronic endometritis, as well as smoking and drug use.
- Fetal factor — caused by a decrease in the proteolytic properties of the blastocyst, which disrupts its normal nidation (implantation) in the upper parts of the uterus.
What obstetric pathologies require differential diagnosis for bleeding in placental previa?
Differential diagnosis of bleeding in placental previa must be carried out with other causes of bleeding in the second half of pregnancy and labor.
The most frequent obstetric pathologies include:
- Placental abruption with normally positioned placenta (abruption placentae).
- Placental accreta (placenta accreta).
- Rupture of fetal vessels (vasa previa) (vasa praevia).
Bleeding should also be differentiated from rare causes associated with changes in the cervix and vagina: ectopias, polyps, cervical and vaginal cancer, as well as ruptured varicose veins of the vagina.
What fetal complications are characteristic of pregnancy complicated by placental previa?
Pregnancy complicated by placental previa is characterized by a number of fetal complications depending on the severity of the mother's condition.
- Acute fetal hypoxia — develops during profuse bleeding against the background of maternal anemia or hemorrhagic shock.
- Fetal malposition — transverse or oblique lie frequently develops.
- Breech presentation — occurs as an indirect sign of pathology alongside a high-standing presenting part.
- Preterm birth — placenta previa is a significant risk factor for its onset.
What are the exact ultrasound criteria for diagnosing placenta previa?
The exact ultrasound criterion for diagnosing placenta previa is the placental edge located less than 3 cm from the internal os or completely/partially covering it.
Assessment is carried out starting from the 20th week of pregnancy. The following criteria by degree are distinguished:
- Grade I — the placental edge is in the lower segment but does not reach the internal os.
- Grade II — the lower edge reaches the internal os without covering it.
- Grade III — the edge asymmetrically covers the internal os.
- Grade IV — the placenta symmetrically covers the internal os.
An edge location at a distance of 3 to 5 cm is considered low-lying placenta.
Remember the three conditions for vaginal delivery in placenta previa according to the "SNO" rule: Spontaneous contractions, No bleeding, Open os partially (incomplete previa).
Class and exam questions
Why is routine vaginal examination contraindicated in placenta previa?
Palpation of the cervix and vaginal fornices can provoke further detachment of the placental tissue and cause profuse bleeding. The examination is performed only in a fully prepared operating room if ultrasound is impossible to perform.
Whose blood (maternal or fetal) is lost by a woman during placental abruption with placenta previa?
Only maternal blood is lost. Bleeding occurs from the exposed vessels of the maternal placental site after the chorionic villi detach.
How does amniotomy help stop bleeding in labor?
Amniotomy eliminates the tension of the membranes that pull on the placental edge. In addition, it allows the fetal head to descend into the pelvic cavity and mechanically compress the detached area of the placenta against the uterine wall.
What else to study on this topic
- Topography of anatomical structures in placental abruption with placenta previa
- Infusion therapy regimens and management of massive obstetric blood loss
- Indications and technique for performing mini-cesarean section in the second trimester
- Assessment of occult blood loss and calculation of circulating blood volume (CBV) deficit
- Tocolytic therapy regimens for expectant management
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