Method for obtaining low molecular weight heparins (LMWH) with a weight average molecular weight distribution between 3.0 and 5.0 kDa comprising at least one concentration step by tangential flow filtration (TFF). The method is particularly useful for the preparation of bemiparin and enoxaparin without the use of fractional precipitation nor the use of alcoholic solutions. In particular, the preparation of LMWH is obtained by depolymerization of heparin and filtration (TFF ultrafiltration and/or diafiltration) of the depolymerized heparin without the use of fractional precipitation and without an alcoholic solution.