Underlying mechanisms of retained placenta: Evidence from a population based cohort study

Objective To determine risk factors for retained placenta, and to identify supporting epidemiologic evidence for the three previously-proposed mechanisms: (i) invasive placentation, (ii) placental hypo-perfusion, and (iii) inadequate uterine contractility. Design A retrospective population-based cohort study. Setting and population Israeli population in the southern district. Methods Data were analyzed from a tertiary hospital database, between 1989 and 2014, using univariate tests and generalized estimating equation (GEE) multivariable models. Main outcome measures Prevalence of retained placenta. Results The study population included 205,522 vaginal deliveries of which 4.8% (n = 9870) were complicated with retained placenta. Previous intra-uterine procedures and placenta-related pregnancy complications were found to be significant risk factors for retained placenta (history of cesarean section aOR = 8.82, 95%CI 8.35–9.31; history of curettage aOR = 12.80, 95%CI 10.57–15.50; pre-eclampsia aOR = 1.25, 95%CI 1.14–1.38; delivery of a small for gestational age neonate aOR = 1.08, 95%CI 1.01–1.16; stillbirth aOR = 2.34, 95%CI 1.98–2.77). During labour, the risk for retained placenta was increased in presence of arrest of dilatation (aOR = 2.03, 95%CI 1.08–3.82) or arrest of descent (aOR = 1.55, 95%CI 1.22–1.96). Infections of the uterine cavity during labour were also found to be strongly associated with increased risk of retained placenta (endometritis aOR = 2.21, 95%CI 1.64–2.97; chorioamnionitis aOR = 3.35, 95% CI 2.78–4.04). Conclusions Supporting epidemiologic evidence were found for all three underlying mechanisms. In addition, there is evidence to suggest that intrauterine infection and inflammation may also be a possible pathology associated with retained placenta.