What is placental previa, its definition, risk factors, signs and symptoms, types, management, mode of delivery, and complications?

Placenta Previa: Comprehensive Overview

Definition

Placenta previa occurs when the placenta abnormally implants in the lower uterine segment and overlies the internal cervical os, affecting approximately 1 in 200 pregnancies at delivery. 1, 2

• The condition is characterized by abnormal placental placement that obstructs the cervical opening 3
• Placenta previa totalis represents the most severe form where the placenta completely covers the internal cervical os 3
• Low-lying placenta refers to placental edge within 2 cm of the internal os but not covering it 4
• The condition is common in early pregnancy (42.3% at 11-14 weeks, 3.9% at 20-24 weeks) but most cases resolve by 28 weeks gestation 2

Risk Factors

Prior cesarean delivery is the most significant risk factor, with risk increasing dramatically with each subsequent cesarean section. 1, 4

• Previous cesarean deliveries: Risk increases from 3% with placenta previa and no prior cesarean to 11%, 40%, 61%, and 67% for first, second, third, fourth, and fifth or more cesareans respectively 1
• Advanced maternal age 1, 4, 5
• Multiparity and high gravidity 1, 4, 5
• Prior uterine surgeries including curettage 1, 4, 5
• History of pregnancy termination 4
• Asherman syndrome 1
• Prior placenta previa 5
• Smoking 4, 5
• Multifetal gestation 4
• In vitro fertilization 2

Signs and Symptoms

The hallmark presentation is painless vaginal bleeding in the second or third trimester, though approximately half of cases may have no identifiable bleeding. 1, 3

• Painless vaginal bleeding typically occurring after 20 weeks gestation 1, 3
• "Bloody show" may occur as cervix prepares for labor 1
• Bleeding episodes may be recurrent, with women experiencing one episode at increased risk for subsequent hemorrhage 2, 6
• Contractions may accompany bleeding 3
• Some cases remain asymptomatic until delivery 2

Types/Classification

Placenta previa is classified based on the relationship of the placental edge to the internal cervical os. 4

• Complete (total) placenta previa: Placenta completely covers the internal cervical os 3, 4
• Partial placenta previa: Placenta partially covers the internal os 4
• Marginal placenta previa: Placental edge reaches but does not cover the internal os 4
• Low-lying placenta: Placental edge within 2 cm of internal os but not covering it; vaginal delivery may be safe when distance exceeds 2 cm 4

Diagnosis

Transvaginal ultrasound is the gold standard diagnostic modality with sensitivity of 90.7% and specificity of 96.9%. 2, 4

• Digital pelvic examination must be avoided until placenta previa has been excluded to prevent triggering catastrophic hemorrhage 1, 2, 7
• Initial transabdominal ultrasound followed by transvaginal ultrasound for precise assessment 1, 2, 7
• Transvaginal ultrasound is both safe and accurate for placenta previa diagnosis 1, 2
• Doppler ultrasound essential to identify vasa previa and assess placental blood flow 2
• MRI may be helpful for posterior placenta previa or suspected placenta percreta 2
• All women with placenta previa and prior cesarean deliveries must be evaluated for placenta accreta spectrum disorder 2, 7

Management

Antepartum Management

Women with asymptomatic placenta previa before 28 weeks can continue moderate-to-vigorous physical activity, but after 28 weeks should restrict activity to low-intensity walking and activities of daily living. 2, 6

• Hospitalization is mandatory for women with active bleeding 2, 7
• Strict bed rest during active hemorrhage 7
• Optimize hemoglobin values with oral or intravenous iron supplementation 2, 6
• Serial hemoglobin monitoring and transfusion to prevent maternal decompensation 7
• Antenatal corticosteroids when delivery anticipated before 37 0/7 weeks 2
• Avoid travel requiring moderate-to-vigorous activity after 28 weeks; limit to destinations within 30-60 minutes of level III/IV facilities 6

Delivery Planning

Planned cesarean delivery at 34 0/7 to 35 6/7 weeks gestation is recommended for uncomplicated placenta previa to balance neonatal morbidity against maternal hemorrhage risk. 2

• Delivery must not be delayed beyond 36 0/7 weeks as approximately 50% of women with placenta accreta spectrum beyond 36 weeks require emergent delivery for hemorrhage 2, 6
• Earlier delivery indicated for persistent bleeding, preeclampsia, labor, rupture of membranes, or fetal compromise 2
• Delivery must occur at level III or IV maternal care facility with multidisciplinary team including maternal-fetal medicine, pelvic surgeons, urologists, interventional radiologists, obstetric anesthesiologists, and neonatologists 2, 6
• Blood bank capable of massive transfusion protocols is essential 2, 6
• Preoperative coordination with all subspecialists mandatory 2
• Regional anesthesia for cesarean delivery is safe 4

Intraoperative Management

The surgical approach involves careful planning with uterine incision placed away from the placenta when possible, and if placenta accreta spectrum is encountered, the placenta must be left in situ. 2

• Consider dorsal lithotomy positioning for vaginal access and optimal visualization 2
• Inspect uterus after peritoneal entry to determine placental location and optimize incision site 2
• Make uterine incision away from placenta when feasible 2
• Forced placental removal must be avoided as it causes profuse hemorrhage 2, 6, 4
• For placenta accreta spectrum, cesarean hysterectomy with placenta left in situ is the most accepted approach 2, 4
• Consider ureteric stent placement for suspected bladder involvement 2
• Maintain patient temperature >36°C as clotting factors function poorly at lower temperatures 2
• Re-dose prophylactic antibiotics if blood loss ≥1,500 ml 2

Massive Hemorrhage Protocol

When transfusing for acute hemorrhage, use a fixed ratio of 1:1:1 to 1:2:4 for packed red blood cells:fresh frozen plasma:platelets. 2, 7

• Baseline laboratory assessment should include platelet count, prothrombin time, partial thromboplastin time, and fibrinogen levels 2
• Close monitoring of volume status, urine output, blood loss, and hemodynamics is critical 2
• Intensive hemodynamic monitoring in early postoperative period, often requiring intensive care unit 2
• Maintain low threshold for reoperation if ongoing bleeding suspected 2

Mode of Delivery

Cesarean delivery is mandatory for complete placenta previa; vaginal delivery may be considered only when placental edge-to-internal os distance exceeds 2 cm. 4

• All women with complete placenta previa require cesarean delivery 4
• Vertical uterine incision may be necessary depending on placental location 3
• Timing at 34 0/7-35 6/7 weeks balances neonatal and maternal risks 2
• At periviable gestational ages (22 weeks), delivery decisions must prioritize maternal survival exclusively, as cesarean carries significant maternal morbidity without proven fetal benefit 7

Complications

Maternal Complications

Placenta accreta spectrum disorder is the most serious complication, occurring in 3% of women with placenta previa and no prior cesareans, increasing to 67% with five or more prior cesareans. 1

• Massive hemorrhage is the primary cause of maternal morbidity and mortality 2, 3, 4
• Placenta accreta spectrum disorders with risk increasing 7-fold after one cesarean to 56-fold after three cesareans 2, 7, 6
• Hysterectomy required in majority of women with significant placenta accreta 4
• Need for massive blood transfusion 2, 4
• Maternal mortality risk 3, 5
• Bladder injury in cases of placenta percreta 2
• Coagulopathy from massive blood loss 2
• Postoperative complications requiring intensive care 2

Fetal/Neonatal Complications

Preterm birth is the most common perinatal complication, with increased rates of intrauterine growth restriction and neonatal morbidity. 8

• Preterm birth from indicated early delivery 8, 9
• Intrauterine growth restriction 8
• Increased neonatal morbidity and mortality 8, 9
• Fetal exsanguination if vasa previa coexists 4
• Respiratory distress syndrome from prematurity 8

Common Pitfalls

Failure to diagnose placenta accreta spectrum in women with placenta previa and prior cesarean deliveries can lead to catastrophic hemorrhage. 2

• Performing digital examination before excluding placenta previa triggers massive bleeding 1, 2, 7
• Attempting manual placental removal when accreta encountered causes profuse hemorrhage 2, 6
• Delaying delivery beyond 36 weeks increases emergency delivery risk to 50% 2, 6
• Delivering at facilities without adequate blood banking and multidisciplinary expertise 2, 6
• Inadequate preoperative hemoglobin optimization 2, 6
• Failure to have massive transfusion protocol in place 2, 7