Coagulation is a normal response to blood vessel injury and involves the interaction of endothelium, platelets and clotting factors. Coagulation is altered by pregnancy and may be further altered by thrombophilia, an acquired or inherited predisposition to develop thrombosis. An overview of coagulation is provided as background for understanding thrombophilia. Both acquired and genetic risk factors for thrombosis are discussed. Thrombosis may affect not only the maternal circulation, but the utero-placental-fetal circulation as well. The literature documenting the association between maternal thrombosis and thrombophilia is summarized, as is the recent data linking thrombophilia and poor pregnancy outcome. An approach to screening for thrombophilia is outlined and strategies for thromboprophylaxis are provided.