Antithrombotic therapy is often used during pregnancy for the treatment and prevention of venous thromboembolism, systemic embolism in patients with heart valve prostheses, for prevention of foetal loss in patients with antiphospholipid syndrome, placenta-mediated complications. In common, low molecular weight heparins (LMWHs) have largely replaced unfractionated heparin as the anticoagulant. However, in case of placenta-mediated complications it has off-label status. The effectiveness and safety of the LMWHs in pregnant women at an increased risk of placenta-mediated complications is discussed in this review. LMWH is effective, safety, easy to administer and associated with a low incidence of foetal and maternal complications.