Increased risk of miscarriage or stillbirth in pregnant women with inherited thrombophilia is small

Research published this week in PLoS Medicine finds that pregnant women with an inherited condition that makes them more likely to form blood clots only face a small increase in the risk that they might have a miscarriage or stillbirth. The findings also show that there is no significant association between this condition, known as inherited thrombophilia, and placenta-related problems in pregnancy, suggesting that the prescription of drugs to women with thrombophilia to prevent these problems would be premature.

Marc Rodger, of the University of Ottawa in Canada, and colleagues conducted a systematic review of studies investigating placenta-related problems in pregnancy and women with thrombophilia. More than one in twenty pregnancies have complications that are linked to the placenta, These complications include pregnancy loss, pre-eclampsia (high blood pressure), pregnancies that result in the baby not growing properly in the womb, and problems occurring when the placenta becomes separated from the womb.

Previous retrospective studies suggested that there was a link between the two most common inherited thrombophilias, factor V Leiden (FVL) and prothrombin gene mutation (PGM), and placenta-related pregnancy problems. These studies were conducted by analyzing databases of pregnancy records and comparing outcomes for women with and without inherited thrombophilias. By undertaking this new systematic review of prospective studies, which are conducted by recruiting patients (pregnant women with and without inherited thrombophilias) and following them over time, Rodger and colleagues were able to estimate the risk of pregnancy complications in women with FVL or PGM.

The researchers found that the increase in the risk of pregnancy loss in women with FVL was only 1%, whilst there was no significant increase in the risk to women with PGM. The findings also show no association between inherited thrombophilia and other placenta-related pregnancy problems.. "The negative findings described in this publication are important", say the researchers, because previous studies have led some clinicians to prescribe anticoagulant drugs to pregnant women with thombophilia. Further research is required, they conclude, but the prescription of anticoagulant drugs is "premature and should be considered experimental".