The main aim of this study is to compare the influence of total body-weight (TBW), lean body-weight (LBW), ideal body-weight (IBW) or Body Mass Index (BMI) on peak anti-Xa levels at steady-state in bariatric surgery (BS) morbidly obese patients receiving a fixed double dose of dalteparin (5000 IU subcutaneously, twice daily a day) for venous thromboembolism (VTE) prevention. Secondary aims of this study are to identify which body size descriptors (BSD) has the greatest impact and could further be explored for an adjusted low molecular weight heparin (LMWH) dosing algorithm for achieving optimal VTE prevention in BS severely obese patients.