# Placenta Previa, Accreta Spectrum, and Vasa Previa

## Placenta Previa

### Definition and Classification

Placenta previa occurs when the placenta partially or completely covers the internal cervical os. Current terminology has been simplified to two categories: placenta previa, in which placental tissue covers the internal os, and low-lying placenta, in which the placental edge is within 2 cm of the internal os but does not cover it. The older terms "partial" and "marginal" previa are no longer standard.

### Epidemiology and Risk Factors

The incidence of placenta previa at delivery is 0.3 to 0.5% of pregnancies. Previa is more commonly identified on second-trimester ultrasound because approximately 90% of previas seen at that time will resolve as the lower uterine segment elongates during the third trimester. Posterior previas are more likely to resolve than anterior previas. The strongest risk factor for placenta previa is prior cesarean delivery, with risk increasing with each additional cesarean. Other risk factors include prior uterine surgery (myomectomy, D&C), multiparity, advanced maternal age, smoking, cocaine use, multiple gestation, IVF, and a history of prior placenta previa.

### Clinical Presentation

The hallmark of placenta previa is painless vaginal bleeding, typically occurring in the second or third trimester. The initial episode, often called a sentinel bleed, is frequently self-limited. However, some patients present with massive hemorrhage. The first bleeding episode usually occurs around 28 to 30 weeks' gestation. Some patients with previa remain entirely asymptomatic and are diagnosed only on routine ultrasound.

### Diagnosis

Transvaginal ultrasound (TVS) is the gold standard for diagnosing placenta previa and is safe -- the probe does not enter the cervical canal. Transabdominal ultrasound may miss posterior previas. TVS accurately determines the relationship between the placental edge and the internal os.

### Management

For an asymptomatic previa discovered at the 18 to 22 week anatomy scan, follow-up TVS at 32 weeks is recommended to reassess. Most will have resolved by this time. For a persistent previa in the third trimester, management includes pelvic rest (no intercourse, no vaginal examinations), activity restriction based on clinical judgment, hospitalization for any active bleeding episode, RhoGAM for Rh-negative patients with bleeding, and maintenance of a current type and screen with crossmatch-ready blood available. Regarding delivery timing, uncomplicated previa is delivered by scheduled cesarean at 36 weeks 0 days to 37 weeks 6 days. When accreta spectrum is also suspected, delivery is moved earlier to 34 to 35 weeks. Emergent delivery is indicated at any gestational age for life-threatening hemorrhage. Antenatal corticosteroids should be administered if delivery is anticipated before 37 weeks.

<image>Transvaginal ultrasound image showing complete placenta previa with the placenta entirely covering the internal cervical os, with labeled anatomical landmarks including the bladder, cervix, internal os, and placental tissue</image>

## Placenta Accreta Spectrum (PAS)

### Classification

Placenta accreta spectrum disorders represent a continuum of abnormal placental invasion. In placenta accreta (accounting for 75 to 80% of PAS cases), the chorionic villi attach directly to the myometrium without the normal intervening decidua basalis. In placenta increta (approximately 15% of cases), the villi invade into the myometrium. In placenta percreta (approximately 5% of cases), the villi penetrate through the entire myometrium to or beyond the serosa, potentially invading adjacent organs, most commonly the bladder.

| PAS Type | Depth of Invasion | Frequency |
|---|---|---|
| Accreta | Villi attach to myometrium (no decidua basalis) | 75-80% |
| Increta | Villi invade into myometrium | ~15% |
| Percreta | Villi penetrate through serosa (± adjacent organs) | ~5% |

### Epidemiology

The incidence of PAS is approximately 1 in 500 to 1 in 2,500 deliveries and is increasing in parallel with rising cesarean delivery rates. The single greatest risk factor is a placenta previa overlying a prior uterine scar, and the risk escalates dramatically with each additional cesarean: previa with no prior cesarean carries a 3% accreta risk, previa with one prior cesarean carries an 11% risk, previa with two prior cesareans carries a 40% risk, and previa with three prior cesareans carries a 61% risk. Additional risk factors include other prior uterine surgery, advanced maternal age, multiparity, and IVF.

| Placenta Previa + Prior Cesareans | Accreta Risk |
|---|---|
| Previa, no prior cesarean | ~3% |
| Previa + 1 prior cesarean | ~11% |
| Previa + 2 prior cesareans | ~40% |
| Previa + 3 prior cesareans | ~61% |

### Prenatal Diagnosis

Ultrasound findings suggestive of PAS include loss of the retroplacental clear zone (the normal hypoechoic area between the placenta and myometrium), myometrial thinning or absence overlying the placenta, placental lacunae producing a "Swiss cheese" appearance (this is the most predictive finding), bridging vessels extending from the placenta into the bladder wall, and increased vascularity at the uterine-bladder interface on color Doppler. MRI is used as an adjunctive study when ultrasound is equivocal or when delineation of the extent of invasion is needed, particularly with posterior placenta or suspected parametrial involvement. However, MRI does not reliably differentiate accreta from increta or percreta.

### Multidisciplinary Planning

Delivery of confirmed or highly suspected PAS should occur at a center of excellence with a multidisciplinary team including maternal-fetal medicine, gynecologic oncology or an experienced pelvic surgeon, anesthesiology (with arterial line and large-bore IV access), interventional radiology (balloon occlusion catheters are available but optional), a blood bank with a massive transfusion protocol prepared, neonatal intensive care, and ICU availability. Average blood loss during PAS hysterectomy ranges from 2 to 5 liters, and a cell saver is recommended.

### Surgical Management

Planned cesarean hysterectomy is the standard of care for confirmed PAS. Delivery is timed at 34 weeks 0 days to 35 weeks 6 days to balance the competing risks of prematurity against unplanned hemorrhage from labor or bleeding. Antenatal corticosteroids are administered before delivery. The key surgical principle is to deliver the infant through a uterine incision placed away from the placenta, often a fundal or high transverse hysterotomy. The placenta is left in situ and no attempt is made to remove it, as doing so causes massive, potentially fatal hemorrhage. The procedure then moves directly to hysterectomy. Conservative management -- leaving the placenta in situ with or without methotrexate -- is considered in highly select cases of focal accreta when fertility preservation is paramount, but it carries risks of hemorrhage, infection, DIC, and the need for delayed hysterectomy.

<image>Cross-sectional diagram of the uterine wall showing the spectrum of abnormal placentation: normal placentation with decidua basalis intact, placenta accreta with villi attached to myometrium, placenta increta with villi invading into myometrium, and placenta percreta with villi penetrating through serosa into the bladder</image>

## Vasa Previa

### Definition

Vasa previa occurs when fetal blood vessels traverse the fetal membranes over or near the internal cervical os, unprotected by either placental tissue or Wharton's jelly. In Type 1 vasa previa, a velamentous cord insertion sends vessels across the os. In Type 2, vessels connecting a succenturiate (accessory) placental lobe to the main placenta cross the os.

### Epidemiology

The incidence is approximately 1 in 2,500 to 1 in 5,000 deliveries. Risk factors include velamentous cord insertion, bilobed or succenturiate placenta, a low-lying placenta in the second trimester, IVF pregnancies, and multiple gestation.

### Clinical Significance

The clinical significance of vasa previa is enormous because rupture of the membranes -- whether spontaneous or artificial -- can tear the unprotected fetal vessels. Because the fetal blood volume at term is only approximately 250 mL, hemorrhage from torn fetal vessels is rapid and catastrophic. When vasa previa is undiagnosed, fetal mortality rates range from 50 to 95%. When diagnosed prenatally and managed with a planned approach, survival rates exceed 95%.

### Diagnosis

The diagnosis is made by transvaginal ultrasound with color Doppler, which visualizes fetal vessels crossing the internal os. It is best identified during the second-trimester anatomy scan or on follow-up imaging for a low-lying placenta. Pulsed-wave Doppler confirms the fetal origin of the vessels by demonstrating a fetal heart rate pattern.

### Management

Once diagnosed, serial surveillance is initiated. Many institutions recommend hospitalization at 30 to 34 weeks, though this varies by institution. Antenatal corticosteroids are administered at 32 to 34 weeks in anticipation of preterm delivery. A planned cesarean delivery is performed at 34 to 36 weeks, before the onset of labor and rupture of membranes. Emergent cesarean is required if labor begins or membranes rupture. Amniotomy and digital cervical examination are absolutely contraindicated.

## Clinical Pearls

Transvaginal ultrasound is safe in the setting of placenta previa and is more accurate than the transabdominal approach. The probe does not enter the cervix.

The combination of placenta previa with a prior cesarean scar should immediately raise suspicion for accreta spectrum. Detailed ultrasound evaluation should be obtained and multidisciplinary delivery planning initiated.

Placental lacunae on ultrasound are the single most predictive finding for placenta accreta spectrum.

Manual removal of the placenta should never be attempted in suspected PAS. This is the most important intraoperative principle -- attempting placental removal causes life-threatening hemorrhage.

Vasa previa is one of the few conditions in obstetrics where prenatal diagnosis dramatically changes fetal survival, from less than 10% to greater than 95%. This underscores the importance of routine screening during the anatomy survey.

During every routine anatomy ultrasound, the sonographer should look for velamentous cord insertion and succenturiate placental lobes, as these are the anatomic precursors to vasa previa.

The risk of accreta increases exponentially with the combination of previa and increasing number of prior cesarean deliveries. This relationship is one of the strongest arguments for judicious use of primary cesarean delivery.

## References

- ACOG Committee Opinion No. 764: Medically Indicated Late-Preterm and Early-Term Deliveries (2019)
- ACOG Obstetric Care Consensus No. 7: Placenta Accreta Spectrum (2018, reaffirmed 2023)
- Society for Maternal-Fetal Medicine (SMFM) Consult Series No. 44: Placenta Accreta Spectrum (2018)
- Jauniaux E et al. Placenta previa and placenta accreta: diagnosis and management. BMJ. 2018;362:k1190
- Oyelese Y, Smulian JC. Placenta previa, placenta accreta, and vasa previa. Obstet Gynecol. 2006;107:927-941
- Ruiter L et al. Systematic review of accuracy of ultrasound in the diagnosis of vasa previa. Ultrasound Obstet Gynecol. 2015;45:516-522