Inclusion criteria  >36 weeks corrected gestational to <17 years old <24 hours after insertion of an untunneled CVC CVC inserted in the internal jugular or femoral vein  Exclusion criteria  Radiologic diagnosis of CADVT in the site of insertion in prior 6 weeks Currently receiving an antithrombotic agent, e.g., LMWH, UFH, warfarin and aspirin, but not UFH at dose to maintain patency of a vascular catheter Presence of clinically relevant bleeding, i.e., hemoglobin decreased ≥2 g/dl in 24 hours, required medical or surgical intervention to restore hemostasis, or in the retroperitoneum, pulmonary, intracranial or central nervous system, in the prior 60 days Surgery in the prior 7 days Major trauma in the prior 7 days Presence of coagulopathy, i.e., INR >2.0, aPTT >50 seconds or platelet count <50 x 10^3/mcL Presence of renal failure, i.e., creatinine clearance <30 mL/min/1.73 m2 Known hypersensitivity to heparin or pork products Laboratory confirmed HIT Current pregnancy or lactation Presence of an epidural catheter Limitation of care Previous enrollment in the CRETE Studies