What are the risk factors for placenta accreta spectrum?

Risk Factors for Placenta Accreta Spectrum

Primary Risk Factors

Prior cesarean delivery combined with placenta previa represents the most critical risk factor for placenta accreta spectrum, with risk escalating exponentially based on the number of prior cesareans. 1

Prior Cesarean Delivery (Dose-Dependent Risk)

• Without placenta previa present, the risk of accreta spectrum increases from 0.3% after one cesarean to 6.7% after five or more cesarean deliveries 1
• Prior cesarean delivery is the most common modifiable risk factor, with risk rising in a dose-dependent manner for each additional cesarean section 1
• The overall incidence has surged from 1 in 2,510 deliveries to 1 in 272 deliveries over four decades, directly paralleling the increase in cesarean delivery rates 1, 2

Placenta Previa (Critical Risk Multiplier)

• Placenta previa is present in more than 80% of all placenta accreta spectrum cases, making it the strongest ultrasonographic association 1, 2
• When placenta previa occurs without any prior cesarean, the baseline risk is approximately 3% 1, 2
• When placenta previa overlies a cesarean scar, risk escalates dramatically 1, 2:
  • 1 prior cesarean → 11% risk
  • 2 prior cesareans → 40% risk
  • 3 prior cesareans → 61% risk
  • ≥5 prior cesareans → 67% risk
• Placenta previa overlying a cesarean scar is present in approximately 49% of all accreta spectrum cases 2

Secondary Uterine Risk Factors

Prior Uterine Surgery

• Myomectomy with endometrial cavity entry significantly increases accreta spectrum risk because it creates full-thickness defects in the endometrial-myometrial interface, similar to cesarean delivery 1, 3
• If placenta previa develops overlying a myomectomy scar, risk escalates dramatically—comparable to the cesarean scar-previa combination 3
• Dilation and curettage (D&C) is independently associated with accreta spectrum with an odds ratio of approximately 2.8 1, 4
• Hysteroscopic surgery carries an odds ratio of 5.7 for subsequent accreta spectrum 4
• Uterine artery embolization demonstrates the highest odds ratio at 44.1 for accreta spectrum development 4
• Asherman syndrome (intrauterine adhesions) is an established risk factor 1

Maternal Demographic Factors

• Advanced maternal age (≥35 years) independently raises the risk of placenta accreta spectrum 1, 2
• Multiparity (multiple prior births) is linked to higher incidence 1, 2
• Body mass index ≥30 is independently associated with accreta spectrum in high-risk populations 5
• Previous postpartum hemorrhage is an independent risk factor 5

Assisted Reproductive Technology

• Current pregnancy via assisted reproductive technology (ART) carries an odds ratio of 4.1 for accreta spectrum occurrence 4
• In vitro fertilization is recognized as an additional risk factor by major guideline societies 2

Pathophysiological Mechanism

The underlying mechanism involves defects in the endometrial-myometrial interface caused by uterine scarring, leading to failure of normal decidualization. 1, 3, 6

• This permits abnormally deep placental villous attachment and trophoblast infiltration into areas of prior uterine injury 1, 3
• Any surgery that breaches the endometrial cavity creates the type of disruption that damages the endometrial-myometrial interface and affects scar tissue development 3
• Extracellular matrix remodeling, dysregulated signaling pathways, and immune and vascular alterations at the maternal-fetal interface contribute to abnormal placentation 6

Risk Stratification in Clinical Practice

Posterior vs. Anterior Placenta Location

• In women with prior cesareans, the exact placental location significantly affects risk: rates range from 5% for one prior cesarean with a posterior low-lying placenta to 63% for three or more prior cesareans with anterior placenta previa 5

Combined Risk Assessment

• A woman with two prior cesareans and an anterior placenta previa faces an estimated 40% risk of accreta spectrum 1
• Clinical risk factors remain equally important predictors even when ultrasound imaging appears normal 1, 2
• Ultrasound findings alone should never be used to rule out placenta accreta spectrum 1, 2

Critical Clinical Caveat

The absence of ultrasound abnormalities does NOT exclude placenta accreta spectrum; clinical risk factor assessment is mandatory and equally predictive. 1, 2 High-risk patients (placenta previa overlying a prior cesarean scar) require preparation for accreta spectrum at delivery regardless of imaging findings, because inter-observer variability and technical limitations can produce false-negative ultrasound results. 2