This chapter develops the hypothesis that some women with a history of recurrent miscarriage (RM) are in a prothrombotic state outside of pregnancy. The causes of RM have been grouped into six main categories: genetic, anatomical, infective, endocrinological, immunological and unexplained. The thrombophilic disorders play a part in the aetiology of recurrent pregnancy loss at various gestations. Haemostasis in vivo is a balancing act between the coagulation and fibrinolytic pathways, and plays a vital role in the establishment and maintenance of pregnancy. Prospective studies have shown an increased prevalence of acquired thrombophilic disorders in women with a history of RM. Women with a history of RM are at greater risk of later pregnancy complications such as pre-eclampsia, fetal growth restriction and intrauterine death. Revised criteria for the diagnosis of Primary antiphospholipid syndrome (PAPS) recognise the obstetric manifestations of antiphospholipid antibodies (aPL), such as a history of pre-eclampsia and preterm labour.