Recommendations for the use of heparin for preventing miscarriage are recently rapidly changing based on evidenced based prospective studies. At present either heparin or low molecular weight heparin (LMWH) is recommended for the antiphospholipid syndrome (APS). However criteria for diagnosing APS have become much stricter. The exact timing of the heparin is still being evaluated since it is not clear if the main therapeutic effect is in inhibition of thrombosis when the heparin could be started at the time in the first trimester when the platelets become thrombophilic or does its main role in improving implantation in which it would be started shortly before or shortly after ovulation. Possibly heparin is superior to LMWH in improving the implantation process though more studies are needed to corroborate or refute this suggestion. At present inherited thrombophilias are not considered a cause of first trimester miscarriage and thus measuring these factors are not recommended. There is no evidence that heparin has any benefit in preventing miscarriage from unexplained causes. Heparin is effective alone and there does not appear to be any extra benefit from adding aspirin (or even aspirin may negate some of its benefits).