For two decades, women at risk of developing placental blood clots have been prescribed the anticoagulant low molecular weight heparin (LMWH) but it's ineffective, according to a clinical trial published today in The Lancet.

As many as one in 10 pregnant women have a tendency to develop  thrombophilia
- blood clots in their vein. These women have been injected with LMWH daily, which means hundreds of needles over the course of their pregnancy. 

A randomized clinical trial led by Dr. Marc Rodger, a senior scientist at the Ottawa Hospital Research Institute who heads up the Thrombosis Program of The Ottawa Hospital, provides conclusive evidence that the commonly prescribed LMWH anticoagulant has no positive benefits for the mother or child. In fact, Dr. Rodger's study shows that LMWH treatments could actually cause pregnant women some minor harm by increasing bleeding, increasing their rates of induced labour and reducing their access to anesthesia during childbirth. 

"These results mean that many women around the world can save themselves a lot of unnecessary pain during pregnancy," says Dr. Rodger, who is also a professor in the Faculty of Medicine at the University of Ottawa. "Using low molecular weight heparin unnecessarily medicalizes a woman's pregnancy and is costly."

Angela Donatucci of Ottawa, Canada is giving herself one of two daily injections of low molecular weight heparin. Note the bruising below and to the right of her belly button. Credit: Ottawa Hospital Research Institute

Since the 1990s, using LMWH to treat pregnant women with a tendency to develop blood clots became commonplace, despite the fact that a large, multi-site randomized clinical trial had never been conducted to prove its effectiveness. Low molecular weight heparin is also prescribed by many physicians worldwide to women, with and without thrombophilia, to prevent placenta blood clots that may lead to pregnancy loss, as well as preeclampsia (high blood pressure), placental abruption (heavy bleeding) and intra-uterine growth restrictions (low birth weight babies). The anticoagulant LMWH is also prescribed to prevent deep vein thrombosis (leg vein blood clots) and pulmonary embolisms (lung blood clots).

"While I wish we could have shown that LMWH prevents complications, we actually proved it doesn't help," adds Rodger. "However, I'm very glad that we can now spare these women all those unnecessary needles."

The clinical trial took 12 years to complete and involved 292 women at 36 centres in five countries. The study's results were published online today and will appear in a future print issue of The Lancet, one of the world's oldest and most prestigious medical journals. It is also the subject of a Comment published online today.

"Dr. Rodger's findings will benefit many women in many countries who will be spared from hundreds of unnecessary and painful injections. They also underscore the importance of conducting rigorous, well-designed clinical trials, something we pride ourselves on at the Ottawa Hospital Research Institute," says Dr. Duncan Stewart, chief executive officer and scientific director of the Ottawa Hospital Research Institute, vice-president of research at The Ottawa Hospital and professor of medicine at the University of Ottawa.

Rodger says he hopes that doctors will stop prescribing LMWH to pregnant women with thrombophilia and/or with previous pregnancy complications when it isn't warranted. He also hopes the results of his study will cause some sober second thought in the medical community and bolster the use of evidence-based treatments.  

"These findings allow us to move on, to pursue other, potentially effective, methods for treating pregnant women with thrombophilia and/or complications from placenta blood clots," says Dr. Rodger.

There is still one type of thrombophilia (anti-phospholipid antibodies) for which blood thinners may be effective in preventing recurrent pregnancy loss. Also, some women are advised to take low-dose aspirin while pregnant to help prevent pregnancy complications. All women with thrombophilia need blood thinners to prevent blood clots after delivery.

As noted in the paper, some women with prior severe pregnancy complications might still benefit from blood thinners, but this requires further rigorous study. However, in all cases, pregnant women experiencing any type of prior complication should consult their doctor about the right course of treatment.