Retained placenta is a common complication of pregnancy affecting 1% to 6% of all births. If a retained placenta is left untreated, spontaneous delivery of the placenta may occur, but there is a high risk of bleeding and infection. Manual removal of the placenta (MROP) in an operating theatre under anaesthetic is the usual treatment, but is invasive and may have complications. An effective non-surgical alternative for retained placenta would potentially reduce the physical and psychological trauma of the procedure, and costs. It could also be lifesaving by providing a therapy for settings without easy access to modern operating theatres or anaesthetics. Injection of uterotonics into the uterus via the umbilical vein and placenta is an attractive low-cost option for this. This is an update of a review last published in 2011. To assess the use of umbilical vein injection (UVI) of saline solution with or without uterotonics compared to either expectant management or with an alternative solution or other uterotonic agent for retained placenta. For this update, we searched Cochrane Pregnancy and Childbirth's Trials Register, ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform (14 June 2020), and reference lists of retrieved studies. Randomised controlled trials (RCTs) comparing UVI of saline or other fluids (with or without uterotonics), either with expectant management or with an alternative solution or other uterotonic agent, in the management of retained placenta. We considered quasi-randomised, cluster-randomised, and trials reported only in abstract form. Two review authors independently assessed trials for inclusion and risk of bias, extracted data, and checked them for accuracy. We assessed the certainty of the evidence using the GRADE approach. We calculated pooled risk ratios (RRs) and mean differences (MDs) with 95% confidence intervals (CIs), and presented results using 'Summary of findings' tables. We included 24 trials (n = 2348). All included trials were RCTs, one was quasi-randomised, and none were cluster-randomised. Risk of bias was variable across the included studies. We assessed certainty of evidence for four comparisons: saline versus expectant management, oxytocin versus expectant management, oxytocin versus saline, and oxytocin versus plasma expander. Evidence was moderate to very-low certainty and downgraded for risk of bias of included studies, imprecision, and inconsistency of effect estimates. Saline solution versus expectant management There is probably little or no difference in the incidence of MROP between saline and expectant management (RR 0.93, 95% CI 0.80 to 1.10; 5 studies, n = 445; moderate-certainty evidence). Evidence for the following remaining primary outcomes was very-low certainty: severe postpartum haemorrhage 1000 mL or greater, blood transfusion, and infection. There were no events reported for maternal mortality or postpartum anaemia (24 to 48 hours postnatal). No studies reported addition of therapeutic uterotonics. Oxytocin solution versus expectant management UVI of oxytocin solution might slightly reduce in the need for manual removal compared with expectant management (mean RR 0.73, 95% CI 0.56 to 0.95; 7 studies, n = 546; low-certainty evidence). There may be little to no difference between the incidence of blood transfusion between groups (RR 0.81, 95% CI 0.47 to 1.38; 4 studies, n = 339; low-certainty evidence). There were no maternal deaths reported (2 studies, n = 93). Evidence for severe postpartum haemorrhage of 1000 mL or greater, additional uterotonics, and infection was very-low certainty. There were no events for postpartum anaemia (24 to 48 hours postnatal). Oxytocin solution versus saline solution UVI of oxytocin solution may reduce the use of MROP compared with saline solution, but there was high heterogeneity (RR 0.82, 95% CI 0.69 to 0.97; 14 studies, n = 1370; I² = 54%; low-certainty evidence). There were no differences between subgroups according to risk of bias or oxytocin dose for the outcome MROP. There may be little to no difference between groups in severe postpartum haemorrhage of 1000 mL or greater, blood transfusion, use of additional therapeutic uterotonics, and antibiotic use. There were no events for postpartum anaemia (24 to 48 hours postnatal) (very low-certainty evidence) and there was only one event for maternal mortality (low-certainty evidence). Oxytocin solution versus plasma expander One small study reported UVI of oxytocin compared with plasma expander (n = 109). The evidence was very unclear about any effect on MROP or blood transfusion between the two groups (very low-certainty evidence). No other primary outcomes were reported. For other comparisons there were little to no differences for most outcomes examined. However, there was some evidence to suggest that there may be a reduction in MROP with prostaglandins in comparison to oxytocin (4 studies, n = 173) and ergometrine (1 study, n = 52), although further large-scale studies are needed to confirm these findings. UVI of oxytocin solution is an inexpensive and simple intervention that can be performed when placental delivery is delayed. This review identified low-certainty evidence that oxytocin solution may slightly reduce the need for manual removal. However, there are little or no differences for other outcomes. Small studies examining injection of prostaglandin (such as dissolved misoprostol) into the umbilical vein show promise and deserve to be studied further.