Risk Factors and Pathophysiology of Placenta Percreta
Primary Risk Factors
Prior cesarean delivery is the single most important risk factor for placenta percreta, with risk escalating dramatically from 0.3% after one cesarean to 6.74% after five or more cesarean deliveries 1. This dose-dependent relationship directly explains the 13-fold increase in placenta accreta spectrum disorders since the early 1900s, paralleling the rising cesarean delivery rate 2.
The combination of placenta previa with prior cesarean delivery creates the highest risk scenario, with placenta accreta spectrum risk increasing from 3% with no prior cesarean to 11% with one prior cesarean, 40% with two, 61% with three, and 67% with five or more cesarean deliveries 1, 3. This represents a 7-fold to 56-fold increased risk depending on the number of prior cesareans 3.
Additional Established Risk Factors
- Advanced maternal age is an independent risk factor across multiple studies 1, 4
- Multiparity increases risk independent of cesarean history 1
- Prior uterine surgeries or curettage damage the endometrial-myometrial interface 1, 4
- Asherman syndrome (intrauterine adhesions) is an established risk factor 1, 4
- In vitro fertilization increases risk 4
Rare Presentations
Placenta percreta can occur in nulliparous women with unscarred uteri, though this is extremely rare and represents a diagnostic challenge 1, 5, 6. These cases demonstrate that the pathophysiology is not entirely explained by uterine scarring alone 1.
Pathophysiology
The leading hypothesis is that defects in the endometrial-myometrial interface from uterine scarring lead to failure of normal decidualization, allowing abnormally deep placental anchoring villi and trophoblast infiltration 1, 3. The normal decidual separation zone is absent or deficient, preventing the placenta from separating through its normal plane 7.
Mechanism of Invasion
- In placenta percreta specifically, the placental villi invade completely through the myometrium to the uterine serosa and potentially into surrounding organs (bladder, cervix, parametria) 1, 8, 9
- Disruptions within the uterine cavity cause damage to the endometrial-myometrial interface, affecting scar tissue development and creating focal areas where decidualization fails 1
- The placenta adheres directly to myometrial smooth muscle fibers without intervening decidua, making separation impossible without catastrophic hemorrhage 1, 3, 7
Histologic Features
Pathologic examination reveals decidual deficiency with increased extravillous trophoblast at sites of abnormal invasion 1. In delivered placentas, basal plate myometrial fibers may be present, representing focal adherence and confirming the absence of normal decidual separation 1, 7.
Clinical Implications
Placenta percreta carries the highest maternal morbidity of the placenta accreta spectrum, with severe maternal morbidity in 82.1% and mortality in 1.4% of cases 8. The deep pelvic invasion in grade 3E disease (percreta with surrounding organ involvement) creates potential for catastrophic hemorrhage during attempted delivery or hysterectomy 8.
Approximately 50% of women with placenta accreta spectrum who wait beyond 36 weeks require emergent delivery for hemorrhage, which drives the recommendation for planned delivery at 34-36 weeks at a level III or IV maternal care facility with multidisciplinary team capabilities 3.
Critical Pitfall
Attempting manual removal or forced placental delivery when percreta is present causes catastrophic hemorrhage and must be avoided 3, 7, 8. The placenta should be left in situ with planned cesarean hysterectomy as the standard approach 3.