Hazards of heparin: allergy, heparin-induced thrombocytopenia and osteoporosis.

Heparin is the commonest mode of thromboprophylaxis used in pregnancy. It does not cross the placenta but has potential adverse effects on the mother, of which the most important is heparin-induced osteoporosis. The hazards of heparin, including bleeding, skin reactions, heparin-induced thrombocytopenia and osteoporosis are discussed and the relevant literature reviewed. Low-molecular-weight heparins have certain advantages over standard unfractionated heparins, especially in obstetrics. Their longer half-life and increased bioavailability enable once-daily injections, making them more convenient and acceptable. They are as effective as standard heparin but have a theoretically more favourable side-effect profile, providing less anticoagulant relative to antithrombotic activity. Current evidence suggests a lower incidence of heparin-induced thrombocytopenia. A reduced risk of osteoporosis is suggested but not yet proven. Although thrombo-embolism is currently the leading cause of maternal mortality in the UK, antenatal heparin prophylaxis is not given to all women with previous thrombo-embolism because of continued fears concerning heparin-induced osteoporosis. A protocol is presented with guidelines for different levels of obstetric prophylaxis depending on the perceived level of risk.