Retained Placenta: Condition, Definition, and Prevalence
A woman with retained placenta in two separate deliveries most likely has a history of retained placenta itself as the underlying condition—this is the strongest independent risk factor for recurrence, with an adjusted odds ratio of 9.8 for experiencing retained placenta again in a subsequent vaginal delivery 1.
Definition of Retained Placenta
Retained placenta is defined as failure of the placenta to deliver within 30 minutes following fetal delivery 2. This can manifest as:
- Excessive bleeding <30 minutes after delivery without placental separation
- Placenta not separated 30 minutes after delivery
- Confirmed retained placental tissue >2 hours postpartum 3
The condition exists on a spectrum from simple retention to placenta accreta spectrum (abnormally adherent or invasive placenta), which includes accreta (adherent to myometrium), increta (invading into myometrium), and percreta (perforating through uterine wall) 4.
Prevalence
The overall prevalence of retained placenta is 2.8-7.0% in nulliparous women after vaginal delivery, with the higher figure representing confirmed cases meeting strict diagnostic criteria 3. In the general population, the rate is approximately 2.0% at second delivery 5.
Recurrence Risk Pattern
The recurrence risk is substantial and follows a clear pattern:
- Overall recurrence rate: 13.8% among women with a previous retained placenta 6
- 9.3% risk at the immediately subsequent delivery
- 6% risk in later deliveries 6
- Another study found 25.3% recurrence in subsequent vaginal delivery versus only 5.3% in controls 3
Complete (versus partial) retained placenta in the index delivery dramatically increases recurrence risk with an adjusted OR of 4.90 6.
Modifiable Risk Factors for Recurrence
Women with a history of retained placenta face increased recurrence risk with:
- Induction of labor: OR 1.61 in index delivery, OR 1.46 in subsequent delivery 6
- Epidural analgesia: OR 1.45 in index delivery, OR 1.82 in subsequent delivery 6
Important Clinical Distinction
While placenta accreta spectrum is a separate and more serious condition that can cause retained placenta, it has different primary risk factors:
- Previous cesarean section is the dominant risk factor for accreta spectrum (OR 1.96 overall, increasing to 55.9 for three or more cesareans) 4
- The risk of accreta spectrum increases from 1.7 per 10,000 overall to 577 per 10,000 in women with both previous cesarean and placenta previa 4
- Other uterine surgery (curettage, manual removal of placenta, myomectomy) also increases accreta risk (aOR 3.40) 4
However, anterior placental location does not mediate the increased risk of retained placenta in women with previous cesarean section 5, suggesting the mechanisms differ between simple retained placenta and accreta spectrum.
Critical Clinical Implications
A retained placenta event serves as a sentinel marker for subsequent deliveries 6. This is a serious obstetric complication that can cause life-threatening postpartum hemorrhage and requires immediate mobilization of the obstetric team, anesthesiologist, and blood bank 2. The maternal mortality from severe forms (percreta) was historically as high as 7%, though recent data show improvement with proper screening and specialist center delivery 4.
Women with a history of retained placenta should be counseled about their substantially elevated recurrence risk and the modifiable factors (avoiding unnecessary induction, considering epidural risks) that may influence outcomes in future pregnancies.