While the risk of DVT and resulting pulmonary embolism may be real (as apx068 points out -- my condolences!), we need to make sure not to go overboard. Heparin, and even the "safer" low-molecular weight heparins like dalteparin, are not trivial and shouldn't be played around with. Even though dalteparin may be safer than heparin itself, it still comes with risk of bleeding complications. The decision to prophylax someone on dalteparin is complicated. If you are undergoing hip replacement or knee replacement surgery, anticoagulation may be indicated since the chance of getting a dvt is between 50%-80% without prophylaxis. The risk of DVT from flying is nowhere near 50-80%, unless you are at high-risk for DVT to begin with. So the question becomes whether you want to eliminate the very small risk of DVT from flying while giving yourself a small risk of something like hemorrhagic stroke or serious internal bleeding? Of course, I don't know what your specific risk factors are, so please speak with your physicians. Self-medicating can be dangerous to your health.