Anticoagulation therapy for serious blood clots during pregnancy

Pregnant women are more susceptible than non-pregnant women to forming blood clots in their veins (venous thrombosis). When these clots occur in the deep leg veins, the clot can break up and fragments (emboli) move to the lungs where they may block the blood flow to the lungs (pulmonary embolism). This can have serious consequences. Anticoagulants are used to treat clots and are given to pregnant women with increased susceptibility to clotting. These medications thin the blood to reduce the risk of the further thrombosis and reduce the risk of pulmonary embolism. An important complication of treatment is haemorrhage. During pregnancy heparin is the most common anticoagulant used, either the older unfractionated heparin (UFH) or the newer low molecular weight heparin (LMWH). Neither of these cross the placenta, and both have been shown to be safe during pregnancy, whereas there are concerns that warfarin may affect the fetus. LMWH has been shown to be more effective than UFH outside pregnancy.

In this review we planned to compare these two types of heparin in pregnancy for their ability to reduce clotting and their safety. We could not find any randomised controlled trials for inclusion in the review.

This means that we do not have any evidence from randomised controlled trials on the effectiveness of anticoagulation for deep vein thrombosis in pregnancy, and the effectiveness of LMWH compared to UFH. There is a need for further studies in this area.

Authors' conclusions: 

There is no evidence from randomised controlled trials on the effectiveness of anticoagulation for deep vein thrombosis in pregnancy. Further studies are required.

Read the full abstract...

Thromboembolic complications are much higher in pregnancy due to procoagulant changes. Heparin does not cross the placenta and the use of unfractionated heparin (UFH) is the current established practice in prophylaxis and treatment for thromboembolism in pregnancy.


To compare the effectiveness of anticoagulant therapies for the treatment of deep vein thrombosis in pregnancy. The anticoagulant drugs included are UFH, low molecular weight heparin (LMWH) and warfarin.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (March 2010) and reference lists of retrieved studies.

Selection criteria: 

Randomised controlled trials comparing any combination of warfarin, UFH, LMWH and placebo in pregnant women.

Data collection and analysis: 

We used methods described in the Cochrane Handbooks for Systemic Reviews of Interventions for assessing the eligibility of studies identified by the search strategy. A minimum of two review authors independently assessed each study.

Main results: 

We did not identify any eligible studies for inclusion in the review.

We identified three potential studies; after assessing eligibility, we excluded all three as they did not meet the prespecified inclusion criteria. One study compared LMWH and UFH in pregnant women with previous thromboembolic events and, for most of these women, anticoagulants were used as thromboprophylaxis. There were only three women who had a thromboembolic event during the current pregnancy and it was unclear whether the anticoagulant was used as therapy or prophylaxis. We excluded one study because it included only women undergoing caesarean birth. The third study was not a randomised trial.