Severe postpartum haemorrhage (S-PPH) is the leading cause of maternal death worldwide. We aimed to provide an up-to-date summary about S-PPH determinants.

We performed a systematic review following the Preferred Reporting Items for Systematic reviews and Meta-analysis (PRISMA) guidelines. We searched the terms "risk factor", "determinant", "post-partum haemorrhage" and "severe", within Web of Science, Scopus, Science Direct, Medline, Cochrane, and Clinical Trials.gov databases. The primary endpoint was S-PPH, defined as a blood loss ≥1,000 mL a second-line treatment, a PPH-related blood transfusion, a peripartum decrease in haemoglobin level or a PPH-related maternal death. All interventional and observational studies about risks factors for S-PPH published in English or French between January 1, 2004 and August 31, 2018 were eligible. Because studies used very different definitions of S-PPH, meta-analysis was ruled out and we gauged the risk factors using the Bradford Hill's criteria causality framework.

Amongst 1,193 identified abstracts, we selected fourteen studies that collected information on 9,271,519 parturient women (35,825 with S-PPH). Key established determinants of S-PPH were previous PPH, previous caesarean, multiple pregnancy, abnormal placentation, preeclampsia, labour induction, prolonged labour, placental retention, uterine rupture, uterine atony, uterine fibroids, macrosomia, birth canal injuries, instrumental vaginal and caesarean deliveries. Most of associated factors of uncertain significance belonged to socio-environmental characteristics. Biological parameters were all classified as indicators of risk.