Study: Blood Thinners Don’t Prevent Miscarriage

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April 28, 2010 — For women with a history of miscarriages, taking aspirin alone or combined with the blood-thinning drug heparin fails to prevent pregnancy loss, new research shows.

The findings were published in the April 29 issue of the New England Journal of Medicine.

Five percent of women have had two recurrent miscarriages, and 1% have had three recurrent miscarriages, but the causes are unknown.

Aspirin and heparin have been prescribed to women with unexplained recurrent miscarriages in the belief that the miscarriages were related to blood clots developing in the uterine blood vessels.

Aspirin and heparin both thin the blood, reducing the risk of clots. However, there is little evidence on whether this treatment reduces miscarriage.

Blood Thinners and Miscarriage

To test this theory, researchers led by Stef P. Kaandorp, MD, of the University of Amsterdam in the Netherlands, compared 364 women ages 18 to 42 who had histories of unexplained recurrent miscarriages. The women also had no diagnosed uterine disease or antiphospholipid syndrome, an immune system disorder that increases a woman’s risk for blood clots and pregnancy loss.

The women were randomly assigned to one of three treatments: low-dose aspirin with heparin, low-dose aspirin only, or a placebo. Those who received the combination treatment reported more side effects, including bruising, swelling, or itching where the heparin was injected.

The women considered eligible for the study had either not yet conceived or were less than six weeks into their pregnancies. Eventually, 299 women were pregnant during the course of the study; nearly two-thirds of this group successfully gave birth to a live infant. The study took place between 2004 and 2008 at eight hospitals in the Netherlands.

However, live birth rates did not differ between the three treatment groups, suggesting heparin and/or aspirin do not offer any significant benefit:

  • 54.5% of the aspirin-heparin group successfully gave birth.
  • 50.8% of the aspirin-only group successfully gave birth.
  • 57% of the placebo group successfully gave birth.

Better Prevention Needed

In an accompanying editorial, I.A. Greer, MD, of Hull York Medical School in the U.K., said the findings leave doctors and researchers going back to the drawing board.

“The widespread use of antithrombotic interventions for women with two or miscarriages,” Greer writes, “appears to be no more than another false start in the race to identify an effective intervention for this distressing condition that affects so many women.”

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