Recently a study was published in New England Journal of Medicine that revealed shocking facts. I was long time believed that anticoagulant therapy helps prevent unexplained miscarriages in first and second trimester of pregnancy. However, this study revealed that anticoagulant therapy is not helpful at all.

Anticoagulant therapy supposedly prevented miscarriage because anticoagulant itself is a substance that prevents coagulation, which means it prevents blood from clotting (and therefore aids in sustaining healthy pregnancy). Often aspirin or aspirin in combination with heparin was found to be a part of anti-miscarriage anticoagulant therapy.
Study was conducted on a group of women, either trying to conceive or newly pregnant women. Women were randomly placed in groups, receiving either aspirin, aspirin with heparin, or they received placebo treatment.

Then the data about obstetrical complications rates was gathered. This data included among other complications also miscarriage. The study proved that during the study there was no obvious difference among study groups, which means that the group that received aspirin or the group that received combination of aspirin and heparin, did not improve the chance of sustained pregnancy.

In December 2009 a very similar study was presented at the annual meeting of the American Society of Hematology.

That study found that heparin was not found to be successful in preventing unexplained miscarriages in couples trying to conceive. The reasons may vary, and one of the theories suggests that undiscovered clotting problem might be the cause of miscarriages. However the evidence does not support such claims and further studies will be needed to reveal possible causes of recurrent miscarriages.

The fact is that aspirin alone or aspirin in combination with heparin is still often recommended treatment for women diagnosed with antiphospholipid syndrome, which causes thrombosis (blood cloths in both arteries  and veins) as well as pregnancy-related complications, that include not only miscarriage, but also stillbirth, preterm delivery or severe preeclampsia.

It is important to emphasis that the newest studies do show that anticoagulant therapy (described above) did not show success and was not helpful in couples trying to conceive after unexplained recurrent miscarriages.