Risk Factors for Placenta Accreta Spectrum
The most critical risk factor for placenta accreta spectrum is prior cesarean delivery combined with placenta previa, creating a dramatically escalating risk from 3% with no prior cesarean to 67% with five or more prior cesareans. 1, 2
Primary Risk Factors
Prior Cesarean Delivery
- Previous cesarean delivery is the single most common and important modifiable risk factor, with risk increasing in a dose-dependent manner with each additional cesarean 1, 2
- In women without placenta previa: risk increases from 0.3% with one prior cesarean to 6.74% with five or more cesareans 1, 2
- The rising incidence of placenta accreta spectrum over the past four decades (from 1 in 2,510 to 1 in 272 deliveries) directly parallels the increased cesarean delivery rate 1, 2
Placenta Previa: The Critical Multiplier
- Placenta previa is present in more than 80% of placenta accreta spectrum cases, making it the most important ultrasonographic association 1
- When placenta previa occurs without prior cesarean: 3% risk of accreta spectrum 1, 2
- When placenta previa overlies a cesarean scar, risk escalates dramatically 1, 2:
- One prior cesarean: 11%
- Two prior cesareans: 40%
- Three prior cesareans: 61%
- Five or more prior cesareans: 67%
- Placenta previa overlying a cesarean scar is present in 49% of all accreta spectrum cases 2
Additional Established Risk Factors
Other Uterine Surgery and Instrumentation
- Prior uterine surgeries including myomectomy with endometrial cavity entry significantly increase risk 1, 2, 3, 4
- Prior curettage procedures (dilation and curettage) are independently associated with placenta accreta spectrum (OR 2.8) 1, 2, 5
- Hysteroscopic surgery carries substantial risk (OR 5.7) 5
- Uterine artery embolization has the highest odds ratio among surgical interventions (OR 44.1) 5
- Asherman syndrome (intrauterine adhesions) is an established risk factor 1, 2
Maternal Characteristics
- Advanced maternal age (≥35 years) is an independent risk factor 1, 2, 6
- Multiparity increases risk 1
- BMI ≥30 is independently associated with placenta accreta spectrum in high-risk populations 7
Pregnancy-Related Factors
- In vitro fertilization and assisted reproductive technology increase risk (OR 4.1) 2, 5
- Prior postpartum hemorrhage is associated with increased risk 7
- Short interpregnancy interval (<18 months after cesarean) increases risk (OR 6.3) in women with single prior cesarean 4
- Smoking is associated with increased risk (OR 5.8) in women with single prior cesarean 4
Pathophysiology Context
The underlying mechanism involves defects in the endometrial-myometrial interface leading to failure of normal decidualization in areas of uterine scarring, which allows abnormally deep placental anchoring villi and trophoblast infiltration 1, 3, 8. This explains why any disruption within the uterine cavity that damages this interface—whether from cesarean delivery, curettage, or myomectomy entering the endometrial cavity—increases the likelihood of placenta accreta 1, 3.
Critical Clinical Pitfalls to Avoid
- Do not rely solely on ultrasound findings: clinical risk factors remain equally important as predictors even when ultrasound appears normal 1, 8
- Recognize the exponential risk with combined factors: a woman with two prior cesareans and anterior placenta previa has a 40% risk—this is not a low-risk scenario 1, 2
- Posterior low-lying placenta carries lower risk (5% with one prior cesarean) compared to anterior placenta previa, allowing for risk stratification 7
- Early identification is critical: approximately 50% of women with accreta spectrum who wait beyond 36 weeks require emergent delivery for hemorrhage 2