Heparins, unfractionated heparin, and low molecular weight heparin, are the preferred anticoagulants in pregnancy. There are circumstances, however, in which an alternative to heparin should be considered. These circumstances include, the presence of heparin resistance, a heparin allergy manifesting as heparin-induced skin reactions or heparin-induced thrombocytopenia, and the presence of a mechanical heart valve. From time to time, the obstetrician is called on to make recommendations about anticoagulants in pregnancy, including in circumstances in which an alternative to heparin has been suggested or is necessary. In this article, these circumstances are reviewed and alternative anticoagulants are discussed.