Heparin is an injectable blood thinner. Doctors sometimes prescribe heparin to women with antiphospholipid syndrome or other blood-clotting disorders that are linked to recurrent pregnancy loss.
Recurrent miscarriages occur due to a number of causes. Some causes are more well understood than others, and in many cases, there is no known cause for recurrent miscarriages. One of the few medical factors doctors agree upon in recurrent miscarriages is thrombophilia disorders, or medical conditions in which the blood has an increased tendency to clot. The thrombophilia disorder most clearly tied to miscarriages is antiphospholipid syndrome.
In thrombophilia disorders, researchers believe that tiny clots get stuck in the developing placenta, blocking flow of nutrients to the baby and eventually causing miscarriage (or increasing risk of other pregnancy complications, such as pre-eclampsia). It's also been theorized that thrombophilia disorders might cause other problems with the placenta.
For women with diagnosed thrombophilia conditions and recurrent miscarriages, a common treatment is heparin -- often alongside low-dose "baby" aspirin. Heparin injections thin the blood and decreases the tendency to form clots. Evidence suggests that heparin treatment during pregnancy reduces miscarriage rates for women with antiphospholipid syndrome and possibly benefits women with inherited thrombophilia disorders, such as Factor V Leiden mutations.
Using heparin during pregnancy is not without risk; the drug can have side effects in some people and may increase the risk of bone loss or tendency to hemorrhage. But for women with antiphospholipid syndrome, the benefits likely outweigh the risks.
Some experts had theorized that heparin might be beneficial for women with recurrent miscarriages and negative tests for antiphospholipid antibodies, postulating that unexplained recurrent miscarriages could be due to some unrecognized blood clotting disorder, but a 2010 study found that neither heparin nor low-dose aspirin improved birth rates for these women compared to placebo. Heparin treatment is usually recommended only in women with both a history of miscarriage and a confirmed diagnosis of antiphospholipid syndrome or an inherited thrombophilia disorder.
Di Nisio, M., L.W. Peters, S. Middeldorp, "Aspirin or anticoagulants for the treatment of recurrent miscarriage in women without antiphospholipid syndrome. Cochrane Library 2008. Accessed 29 Feb 2008.
Empson, M., M. Lassere, J. Craig, and J. Scott, "Prevention of recurrent miscarriage for women with antiphospholipid antibody or lupus anticoagulant." Cochrane Library. 2008. Accessed 29 Feb 2008.
Kaandorp SP, Goddijn M, van der Post JA, Hutten BA, Verhoeve HR, Hamulyák K, Mol BW, Folkeringa N, Nahuis M, Papatsonis DN, Büller HR, van der Veen F, Middeldorp S. "Aspirin plus Heparin or Aspirin Alone in Women with Recurrent Miscarriage." N Engl J Med. 2010 Mar 24. [Epub ahead of print].
March of Dimes, "Thrombophilias and Pregnancy." Quick Reference: Fact Sheets. Oct 2006. Accessed 29 Feb 2008.