Heparin is a blood thinner that reduces the chances of blood clots forming in the body. Pregnancy tends to increase the risk of blood clots, posing a danger to both the woman and her baby, so women with a history of clots are advised to take this drug. It is, however, placed in Pregnancy Category C by the U.S. Food and Drug Administration (FDA), meaning it should only be taken when the benefits outweigh the health risks. This is because there have been no studies to show whether it is harmful during pregnancy in humans, though researchers agree that it does not cross the placenta. In general, when doctors prescribe heparin in pregnancy, women are advised to take it, because the minimal risks of this medication tend to be better than the risks of blood clots forming.
Some women suffer from thrombophilia disorders, in which the blood is likely to clot, especially during pregnancy. The main problem with letting this condition go untreated is that small clots may appear in the placenta, cutting off the unborn baby's supply of nutrients and oxygen. The result is often miscarriage or stillbirth, though pre-eclampsia — which can be fatal for both the mother and baby — also can occur. For this reason, injections of heparin in pregnancy may be beneficial and can actually reduce the chances of miscarriage. This is why some women with a history of miscarriages and a confirmed thrombophilia disorder may be given heparin when they initially find out they are pregnant.
Like many drugs, though, there is some risk to taking heparin in pregnancy, at least for the mother. For instance, this medication can increase bone loss in women, a risk that already increases during pregnancy, so pregnant women are advised to use calcium supplements to combat this. In addition, blood thinners can cause hemorrhaging, because the blood becomes much thinner, so patients taking heparin in pregnancy are advised to be closely supervised by their doctor.
This medication is in Pregnancy Category C, because no controlled animal or human studies have been completed to officially find out its risks. It is known, however, that it does not cross the placenta and is not found in breast milk, because the molecular weight is too big. This means heparin in pregnancy and breastfeeding is generally considered safe; as with most medications, though, patients should only use this drug when their doctor agrees it is necessary.