Placenta Previa

Definition

• Placenta previa: placental edge covers the internal os
• Low-lying placenta: placental edge is within 2 cm of the internal os but does not cover it

Note on classification: The older categories of complete, partial, and marginal placenta previa are no longer recommended by current guidelines (NICHD terminology). The clinically relevant distinction is simply whether the placenta covers the os or lies within 2 cm of it.

Incidence

• Affects approximately 0.5% of all third-trimester pregnancies (about 1 in 200 pregnancies at term)
• Diagnosed in ~10% of midtrimester sonograms, but the vast majority resolve before delivery
• Incidence appears to be rising, likely related to increasing cesarean delivery rates

Pathogenesis

• Trophotropism: the placenta grows toward the better-vascularized fundus, leaving distal portions near the lower segment to regress. This explains the phenomenon of "placental migration," which is not true movement but differential atrophy
• Bleeding arises when the lower uterine segment develops and effaces, or when contractions, intercourse, or cervical examination disturb the placental-decidual interface

Risk Factors

Clinical Features

• Bleeding is typically unprovoked, though it may be triggered by intercourse, examination, or labor
• Several small "herald bleeds" often precede major hemorrhage
• In up to 10% of cases there is no bleeding until the onset of labor
• Abnormal fetal lie (transverse or oblique) is more common because the placenta prevents normal fetal polarity

Imaging & Diagnosis

• Transabdominal US tends to overdiagnose low-lying placenta (especially with empty bladder) and is inferior for definitive localization
• TVUS is safe - multiple studies have confirmed no increased risk of hemorrhage from careful TVUS; the probe is observed on-screen during insertion to avoid a dilated cervix
• Translabial imaging with the abdominal probe is a useful alternative if TVUS is unavailable

• Placental edge >20 mm from internal os → report as "normal"
• Placental edge ≤20 mm but not covering the os → report as "low-lying"
• Placental edge covering the os → report as "placenta previa"

Placental Migration

Associated Complications

• Placenta accreta spectrum (accreta/increta/percreta): the most serious associated complication; the risk increases markedly when placenta previa overlies a prior uterine scar
• Vasa previa: umbilical vessels coursing in the membranes over the os; risk of catastrophic fetal exsanguination with membrane rupture
• Abnormal fetal lie
• Preterm delivery

Emergency Management (Acute Bleeding)

1. Do NOT perform digital or speculum vaginal examination until placenta previa has been excluded by ultrasound - disruption of the cervical-placental junction can trigger catastrophic hemorrhage
2. Establish two large-bore IVs; obtain CBC, coagulation studies, type and crossmatch
3. Perform transvaginal ultrasound to confirm diagnosis
4. Obstetric consultation immediately
5. Fetal monitoring
6. If ultrasound is unavailable and significant bleeding occurs: double-setup examination - patient taken to OR with preparations for emergency cesarean delivery, then careful vaginal exam beginning in the fornices (avoiding direct cervical entry)

Expectant (Conservative) Management

• Hospitalization for observation and fetal monitoring
• Antenatal corticosteroids for gestational ages <37 weeks to promote fetal lung maturity
• Blood crossmatched upon admission
• Pelvic rest (no intercourse, no digital examination)
• Serial ultrasounds to monitor placental position and fetal growth

Timing and Mode of Delivery

• Cesarean delivery is mandatory for all true placenta previas
• Confirm persistence of previa by TVUS within 1-2 weeks before planned delivery
• Amniocentesis for lung maturity is not recommended - it imposes unnecessary delay
• Delivery should be conducted at a center with NICU capabilities appropriate for gestational age
• If placenta accreta is present: multidisciplinary team, planned cesarean-hysterectomy via fundal incision (avoiding the placenta), with ample transfusion capability

Prognosis

• Maternal: guarded due to risk of torrential hemorrhage before and during delivery; placenta accreta is the most common indication for emergency peripartum hysterectomy
• Neonatal: morbidity and mortality more than doubled compared to uncomplicated pregnancies, largely due to iatrogenic preterm delivery; outcomes depend on gestational age at delivery and whether antenatal corticosteroids were given
• Higher reported rates of fetal anomalies, neurodevelopmental delay, and sudden infant death syndrome have been observed in some studies, though reasons remain unclear