This chapter about the use of antithrombotic agents during pregnancy is part of the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy: Evidence Based Guidelines. Grade 1 recommendations are strong and indicate that the benefits do, or do not, outweigh risks, burden, and costs. Grade 2 suggests that individual patients' values may lead to different choices (for a full understanding of the grading see Guyatt et al, CHEST 2004; 126:179S-187S). Among the key recommendations in this chapter are the following: for women requiring long-term vitamin K antagonist therapy who are attempting pregnancy, we suggest performing frequent pregnancy tests and substituting unfractionated heparin (UFH) or low molecular weight heparin (LMWH) for warfarin when pregnancy is achieved (Grade 2C). In women with acute venous thromboembolism (VTE), we recommend adjusted-dose LMWH throughout pregnancy or IV UFH for at least 5 days, followed by adjusted-dose UFH or LMWH for the remainder of the pregnancy and at least 6 weeks postpartum (Grade 1C+). In patients with a single episode of VTE associated with a transient risk factor that is no longer present, we recommend antepartum clinical surveillance and postpartum anticoagulants (Grade 1C). In patients with a single episode of VTE and thrombophilia or strong family history of thrombosis and not receiving long-term anticoagulants, we suggest antepartum prophylactic or intermediate-dose LMWH or minidose or moderate-dose UFH, plus postpartum anticoagulants (Grade 2C). In patients with multiple (two or more) episodes of VTE and/or women receiving long-term anticoagulants, we suggest antepartum adjusted-dose UFH or adjusted-dose LMWH followed by long-term anticoagulants postpartum (Grade 2C). For pregnant patients with antiphospholipid antibodies (APLAs) and a history of two or more early pregnancy losses or one or more late pregnancy losses, preeclampsia, intrauterine growth retardation, or abruption, we suggest antepartum aspirin plus minidose or moderate-dose UFH or prophylactic LMWH (Grade 2B). We suggest one of the following approaches for women with APLAs without prior VTE or pregnancy loss: surveillance, minidose heparin, prophylactic LMWH, and/or low-dose aspirin, 75 to 325 mg/d (all Grade 2C). In women with prosthetic heart valves, we recommend adjusted-dose bid LMWH throughout pregnancy (Grade 1C), aggressive adjusted-dose UFH throughout pregnancy (Grade 1C), or UFH or LMWH until the thirteenth week and then change to warfarin until the middle of the third trimester before restarting UFH or LMWH (Grade 1C). In high-risk women with prosthetic heart valves, we suggest the addition of low-dose aspirin, 75 to 162 mg/d (Grade 2C).