Placenta retention is associated with threatened abortion at the early stage of pregnancy

Introduction
Retained placenta (RP) is an obstetric issue complicating around 3% of normal deliveries. Previous studies identified various risk factors for RP. In clinical experience, women with a history of threatened abortion (TA) often are noted to have a higher risk of RP. Yet, limited research has been performed in this area.

Aim
To explore the association between RP and TA.

Material and methods
A retrospective cohort study including 219 women with viable pregnancies who had TA and 219 women who had not experienced TA with no evidence of subchorionic hematoma (SCH) between January 2019 and January 2021 were included. Demographic information and adverse pregnancy outcomes were compared. Women who had TA were further divided into two groups: SCH group and non-SCH group, and their demographic information and adverse pregnancy outcomes were also compared. χ² tests or Mann-Whitney U-tests were used for categorical/continuous variables respectively.

Results
The incidence of RP among women with TA was 25/219, compared to 10/219 in the control group (aOR = 2.33, 95% CI: 1.05–5.13, p = 0.04). Women had TA were at a higher risk of postpartum hemorrhage (aOR = 3.73, 95% CI: 1.68–8.27, p = 0.001). In addition, women with SCH were less likely to develop placenta previa (p = 0.04). However, when adjusted for confounders, the difference was not significant (aOR = 0.16, 95% CI: 0.02–1.26, p = 0.08).

Conclusions
We found that threatened abortion is associated with a higher risk of retained placenta as well as postpartum hemorrhage. Therefore, active management at the third stage of labor was recommended among women with a history of threatened abortion.