creation date: 2026-01-08 23:14
tags: Pathologies
Placenta Previa
Background
Definitions
Placenta previa refers to the complete or partial covering of the internal os of the cervix by the placenta.
Low-lying placenta: edge of the placenta is within 2-3.5 cm from the internal os.
Marginal placenta previa: placental edge is within 2cm of the internal os.
Etiology and Pathophysiology
The cause of placental previa is unknown but there is an association with endometrial damage and uterine scaring. Additionally, there is hypothesis that larger placental surface area due to multiple gestations results in a greater chance of encroachment.
In many cases, low-lying placenta will resolve by the third trimester due to placental migration. This is the process in which the increased blood supply at the fundus results in placental growth in the proximal direction while relatively poor blood supply of the lower segment results in regression and atrophy.
Placental bleeding occurs when uterine contractions or cervical changes occur which causes partial detachment of the placenta.
Risk Factors
- Advanced maternal age
- Multiparity
- Smoking
- Cocaine use
- Prior suction and curettage
- Assisted reproductive technology
- History of c-section(s)
- Prior placental previa
Clinical Presentation
Signs & Symptoms
Initial presentation is typically on routine ultrasound examination at 18-20 weeks gestation.
Findings suggestive of placental previa that will persist until term delivery include:
- Lack of resolution by third trimester
- Extension over os by >2.5 cm
- Posterior previa
- Thick placenta
- Previous c-section
- Conception using assisted reproductive technique
Patients may present with:
- Painless vaginal bleeding (90% of cases)
- Uterine contractions, pain, and bleeding (10% of cases)
Bleeding can occur before 30 weeks of gestation and associated with worse outcomes.
History & Physical Exam
Assess for risk factors and history of bleeding.
Placental previa should be suspected in any patient who presents with vaginal bleeding beyond 20 weeks gestation.
Diagnosis
Criteria
Diagnosis of placenta previa is based on sonographic evidence of echogenic homogeneous placental tissue extending over the internal cervical os on a second or third trimester imaging study.
Transvaginal ultrasound is preferred.
Work-up
Additional ultrasound evaluation
At time of ultrasonography, evaluation for placenta accreta spectrum (PAS) should be performed.
Differential
As placenta previa is often found incidentally during ultrasonography, a diagnosis is made simultaneously. However, presentation with vaginal bleeding during pregnancy may be due to:
- Subchorionic hematoma
- Cervicitis
- Cervical cancer
- Pregnancy Loss (Miscarriage)
- Ectopic pregnancy
- Molar pregnancy
In the third trimester, causes include:
- Labour
- Placental abruption
- Vasa previa
Red Flags / Complications
Maternal complications include:
- Massive blood loss
- Risk of hysterectomy (resulting in infertility)
Neonatal complications primarily relate to preterm delivery:
- Lower birth weight
- Lower APGAR scores
- Increased risk for respiratory distress syndrome
Management
The management of pregnancy complicated by placenta previa is described based on the patient’s status and phase of pregnancy.
Asymptomatic
Monitor placenta position
Ultrasound should check position:
- At 32 week gestation
- At 36 week if placenta edge is <2 cm from internal os at 32 weeks
If edge covers internal os at 36 weeks, c-section is indicated. If it does not cover but is <2 cm, consider c-section.
Reduce risk of bleeding
The following practices should be used to reduce risk of bleed:
- Avoid digital cervical exam
- Avoid sexual activity after 20 weeks of gestation
- Avoid moderate to strenuous exercise, heavy lifting, or standing for prolonged periods (>4 h)
Active Antepartum Bleeding
Monitoring of vitals and stabilization with fluids and/or blood products is indicated.
Investigations such as type and screen, CBC, and coagulation panel should be considered to guide treatment.
Tocolytic drugs and transexamic acid are not recommended.
In severe cases, c-section delivery may be indicated.
Stable Following Bleeding Episode
Following stabilization, the following therapy are indicated:
- Antenatal corticosteroids (if bleeding occurred <34 weeks gestation and delivery in next 7 days is anticipated)
- Correction of anemia (oral or parenteral iron supplementation)
- Rho(D) immunoglobulin (RhoGAM)
Postpartum Hemorrhage
Treatment of postpartum hemorrhage is discussed separately. Briefly, oxytocin is administered and titrated. Transexamic acid may be added prophylactically or in response to inadequate response to oxytocin.
Additional steps may include tourniquets and specific suture techniques.