LMW heparin is preferred over unfractionated heparin because:
LMW heparin inhibit factor Xa only and have a weak effect on thrombin , aPTT, or CT are not prolonged. Heparin acts by activation of antithrombin III. LMW heparin is given subcutaneosly due to better bioavalability.LMW heparin have subcutaneous bioavalability of 70 - 90%. From medical pharmacology padmaja 4th edition Page no 337