What are the ACOG guidelines for diagnosing and managing placenta previa?

ACOG Guidelines for Placenta Previa: Diagnosis and Management

Placenta previa should be diagnosed using transvaginal ultrasound, which is the gold standard imaging modality, and women with confirmed placenta previa require planned cesarean delivery at 34 0/7 to 35 6/7 weeks of gestation at a facility equipped for massive transfusion protocols. 1

Diagnostic Approach

Initial Screening and Imaging

• Perform transabdominal ultrasound first as a screening tool to visualize the placenta, inferior placental margin, and cervix from external to internal os 2
• Follow with transvaginal ultrasound for definitive diagnosis when placenta previa is suspected, as this provides superior accuracy with sensitivity of 90.7% and specificity of 96.9% 1
• Never perform digital pelvic examination until placenta previa has been excluded by ultrasound, as this can trigger catastrophic hemorrhage 2, 1
• Add color Doppler evaluation to identify any vessels overlying the internal cervical os (vasa previa) and assess placental blood flow patterns 2

Classification and Follow-Up

• Placenta previa is diagnosed when the placenta overlies the internal cervical os 1
• Low-lying placenta is defined as placental edge within 2 cm of the internal os but not covering it 3
• If placenta reaches or overlaps the internal os at 18-22 weeks, perform follow-up ultrasound at 28-32 weeks to reassess position, as many cases resolve with advancing gestation 2, 3
• If placenta is ≥2 cm from the internal os at 18-20 weeks, no further follow-up is necessary 3

Risk Assessment for Placenta Accreta Spectrum

Critical Risk Stratification

• All women with placenta previa AND prior cesarean deliveries must be evaluated for placenta accreta spectrum disorder, as this combination dramatically increases morbidity and mortality 2, 1
• The risk increases 7-fold after one prior cesarean and up to 56-fold after three cesarean deliveries 2, 1
• Additional risk factors include: advanced maternal age, high parity, in vitro fertilization, prior myomectomy or D&C, prior postpartum hemorrhage, Asherman syndrome, uterine anomalies, smoking, and hypertension 2

Imaging for Accreta Spectrum

• Ultrasound with color Doppler has sensitivity of 77-97% and specificity of 96-98% for detecting placenta accreta 2
• Consider MRI without IV contrast when ultrasound is equivocal, for posterior placentation, or when obesity limits ultrasound assessment 2
• Do not routinely use gadolinium-based contrast agents, as there is insufficient evidence of benefit and potential fetal risk 2

Management Strategy

Antepartum Management

• Hospitalize immediately for any active bleeding episode 1
• Before 28 weeks with asymptomatic placenta previa, moderate-to-vigorous physical activity is acceptable 1
• After 28 weeks, restrict moderate-to-vigorous activity but allow activities of daily living and walking 1
• Optimize hemoglobin values during pregnancy by treating anemia with oral or intravenous iron 1
• Women with one bleeding episode are at increased risk for subsequent hemorrhage and require closer surveillance 1

Delivery Planning and Timing

Optimal Gestational Age

• ACOG recommends planned cesarean delivery at 34 0/7 to 35 6/7 weeks for uncomplicated placenta previa 1
• Do not delay delivery beyond 36 0/7 weeks, as approximately 50% of women with placenta accreta spectrum beyond 36 weeks require emergent delivery for hemorrhage 1
• Deliver earlier than 34 weeks for persistent bleeding, preeclampsia, labor, rupture of membranes, or fetal compromise 1
• Administer antenatal corticosteroids when delivery is anticipated before 37 0/7 weeks 1

Facility and Team Requirements

• Delivery must occur at a level III or IV maternal care facility with multidisciplinary expertise 1
• Assemble a team including: maternal-fetal medicine subspecialists, pelvic surgeons, urologists, interventional radiologists, obstetric anesthesiologists, and neonatologists 1
• Notify blood bank in advance due to frequent need for large-volume transfusion and ensure massive transfusion protocols are available 1

Intraoperative Management

Surgical Approach

• Make the uterine incision away from the placenta when possible 1
• After fetal delivery, leave the placenta in situ if there is evidence of abnormal placental attachment 1
• Never attempt forced placental removal, as this causes profuse hemorrhage 1, 4
• Consider dorsal lithotomy positioning to allow vaginal access and optimal surgical visualization 1

Management of Placenta Accreta Spectrum

• For confirmed placenta accreta spectrum, the most accepted approach is cesarean hysterectomy with placenta left in place 1, 4
• For suspected bladder involvement, place ureteric stents preoperatively and have urologic surgeons available 1
• Have a contingency plan for emergent delivery, as patients are at increased risk of prepartum hemorrhage as gestational age increases 2

Hemorrhage Management

• Transfuse packed red blood cells, fresh frozen plasma, and platelets in a 1:1:1 to 1:2:4 ratio for massive hemorrhage 1, 4
• Maintain patient temperature >36°C, as clotting factors function poorly at lower temperatures 1
• Re-dose prophylactic antibiotics if blood loss ≥1,500 mL 1
• Obtain baseline labs at bleeding initiation: platelet count, PT, PTT, and fibrinogen 1

Postoperative Care

• Provide intensive hemodynamic monitoring in the early postoperative period, often in an ICU setting 1
• Maintain low threshold for reoperation if ongoing bleeding is suspected 1
• Ensure adequate iron supplementation and follow-up of hemoglobin levels if significant blood loss occurred 1

Special Considerations for Vaginal Delivery

• Vaginal delivery may be attempted when the placental edge is >2 cm from the internal cervical os at term 5, 6
• Cesarean delivery is mandatory when the placenta overlaps the internal os 1, 5
• When distance is <2 cm, vaginal delivery is possible but monitor closely for hemorrhage 6, 7

Common Pitfalls to Avoid

• Failing to perform transvaginal ultrasound for accurate diagnosis, relying only on transabdominal imaging 3
• Missing placenta accreta spectrum in women with anterior placenta previa and prior cesarean deliveries, which can lead to catastrophic hemorrhage 2, 3
• Delaying delivery beyond 36 weeks in an attempt to achieve term gestation, which significantly increases emergency delivery risk 1
• Attempting manual placental removal when accreta is encountered, which causes massive hemorrhage 1, 4