Inclusion Criteria:

• Children, aged 0-18 at time of enrollment
• Recently-diagnosed (i.e., within 30 days of radiologic diagnosis) of first- episode acute venous thrombosis

Exclusion Criteria:

• 1) Any of the following:

  1. history of premature birth, if corrected (i.e., post-conceptional) age < 36 weeks (including gestation) at time of enrollment;
  2. pregnancy at time of enrollment (N.B.: development of pregnancy while on study will constitute post-enrollment exclusion from the study, although any data collected to that time will be retained given the intent-to-treat analytic approach);
  3. known pulmonary embolism complicating this first-episode venous thrombosis;
  4. uncorrected anatomic vascular defect, including May-Thurner anomaly, cervical rib, and atretic IVC;
  5. use of thrombolytic therapy (i.e. tPA) in the treatment of this first-episode venous thrombosis;
  6. chronic inflammatory condition/disease, other than diabetes mellitus;
  7. prior episode of treated VTE;
  8. antiphospholipid antibody persistent at 6 weeks (anticardiolipin IgG or IgM ≥ 30, anti-beta-2-glycoprotein-1 IgG or IgM ≥ 10, or lupus anticoagulant positive by local laboratory parameters, e.g. LA1/LA2 ratio ≥ 1.2 by dRVVT);
  9. combined elevation of factor VIII > 150 IU/dL and D-dimer > 500 ng/mL at diagnosis in patients with veno-occlusive DVT involving the extremities, iliac veins, IVC, subclavian vein, brachiocephalic vein, innominate vein, or SVC;
  10. history of malignancy;
  11. no identifiable clinical risk factor for thrombosis (see Table 1) (i.e., episode is spontaneous);
  12. homozygous factor V Leiden mutation;
  13. homozygous prothrombin 20210 mutation;

  14. protein C, S, or antithrombin ≤ 40 % (IU/dL)

      • OR

• 2) At least three of the following:

  1. first-degree family history of VTE (full sibling or biological parent treated for VTE)

  2. thrombophilia traits at most recent assessment prior to enrollment as follows (each counted individually as one risk factor):

     1. factor V Leiden mutation;
     2. prothrombin 20210 mutation;
     3. protein C below lower limit of normal values for age (local laboratory);
     4. protein S below lower limit of normal values for age (local laboratory);
     5. antithrombin III below lower limit of normal values for age (local laboratory);
     6. lipoprotein(a) ≥ 30 mg/dL;
     7. homocysteine ≥ 14 microM;
     8. factor VIII > 150 IU/dL.