**1. Introduction**

Antiphospholipid syndrome (APS) is a rare systemic autoimmune disease characterized by thrombotic events and obstetric complications in the presence of persistently positive antiphospholipid antibodies (aPLs) [1]. The condition may occur alone, that is primary APS, or in association with other autoimmune diseases, most commonly systemic lupus erythematosus (SLE), and is then referred to as secondary APS. The classification criteria (**Table 1**), developed in 1999 [2] and revised in 2006 [1] include clinical features consisting of thrombosis and/or obstetric morbidity in the presence of laboratory criteria such as lupus anticoagulant (LA), medium-high titer IgG/IgM anticardiolipin antibodies (aCL) and/or anti-β2 glycoprotein I (anti-β2GPI). They are often used, also, as diagnostic tools. Obstetric APS (OAPS) subsets are featured by recurrent early miscarriages, fetal death at or beyond 10 weeks of gestation, and early delivery due to severe preeclampsia or placental insufficiency [3, 4]. The first associations between recurrent pregnancy loss and a circulating anticoagulant later known as LA, date back to 1975 [5], but it was not until 1984 Hughes linked the presence of aCL with recurrent miscarriages

defining APS [6]. Nowadays, OAPS is considered one of the few treatable causes of recurrent loss and represents an important health burden for women of childbearing age and a challenge for the physicians [7]. Management of OAPS is challenging for the physician, as individual women with APS do not have the same obstetric risk profile. In the last decade, the importance to stratifying them based on their laboratory and clinical features has been emphasized to quantify the risk of adverse pregnancy outcome. Many efforts have also been made to adjust therapy according to risk stratification [8]. Moreover, new insights into the pathogenesis and clinical understanding of APS have led to potential new therapeutic approaches [9, 10].

This chapter aims to clarify aspects of pathogenesis, clinical features, risk stratification and therapeutic strategies in OAPS.
