Here is the important thing to know. Having antithrombin 3 is normal and good. That helps you clot normally. Antithrombin III is a protein in the blood that blocks abnormal blood clots from forming and keeps a healthy balance between bleeding and clotting. A DEFICIENCY in it is a problem. That is a congenital or inherited condition. People also will have problems if they have TOO MUCH antithrombin III. Hemophiliacs often have this and it also can be brought on by steroid use. This is the diagnostic testing usually done: https://emedicine.medscape.com/article/954688-workup You need to make sure this has been done. I would talk to your doctor and even possibly repeat if it wasn't done or it was done 20 years ago. If you had this deficiency, you'd like be on blood thinning medication like warfarin. Yes, you'd need to have your kids evaluated too. So, talk to your doctor!