Diagnosis of thrombotic disorders depends on the performance of a panel of laboratory tests for individual components of the coagulation mechanism. Although genetic tests are simple and accurate, most disorders still require functional or immunologic tests for diagnosis. Tests for the most important thrombotic defects are described. These tests may be significantly altered by pregnancy, hormones, thrombotic events, systemic illness, and anticoagulant therapy. These changes make some thrombotic disorders difficult to diagnose during pregnancy. Testing before pregnancy for women with a personal or family history of thrombosis will simplify the interpretation.