People often first notice thrombophilia due to a thrombin defect when a blood clot forms unexpectedly, such as a deep vein thrombosis causing leg swelling, pain, warmth, and redness, or a pulmonary embolism with sudden chest pain and shortness of breath. Some learn about it after unusual clotting in situations that don’t typically trigger clots—like after a short trip, while using estrogen-containing contraception or hormone therapy, or with a family history of early or repeated clots—prompting testing for inherited causes. In pregnancy, first signs of thrombophilia due to a thrombin defect may be recurrent miscarriage or placenta-related complications, leading clinicians to investigate “first signs of thrombophilia due to a thrombin defect” through blood tests and family history.