Evaluating the Risk for Pregnancy-Associated Venous Thromboembolism: A 30-Year Study

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The summary below is from the full report titled "Trends in the Incidence of Venous Thromboembolism during Pregnancy or Postpartum: A 30-Year Population-Based Study." It is in the 15 November 2005 issue of Annals of Internal Medicine (volume 143, pages 697-706). The authors are J.A. Heit, C.E. Kobbervig, A.H. James, T.M. Petterson, K.R. Bailey, and L.J. Melton III.

What is the problem and what is known about it so far?

Doctors have long known that pregnant women and those who have recently given birth are at greater than usual risk for forming blood clots in the leg veins (deep vein thrombosis). These blood clots can break loose and lodge in the lungs, an event known as pulmonary embolism. Doctors usually refer to deep vein thrombosis and pulmonary embolism under the combined term "venous thromboembolism." Pregnancy is usually divided into three 3-month periods, called trimesters. The 3 months following delivery is known as the postpartum period. Each trimester of pregnancy and the postpartum period appear to carry different levels of risk for venous thromboembolism, but the relative level of increased risk in each of these periods is not known.

Why did the researchers do this particular study?

They wanted to find out when pregnant women were most likely to develop venous thromboembolism so that those at high risk for the condition could best be protected during the most dangerous times.

Who was studied?

50, 000 women who lived in Olmsted County, Minnesota, and who were pregnant during a 30-year period from 1966 to 1995 were included in the study.

How did the researchers do the study?

The researchers used the data resources of the Rochester Epidemiology Project, a long-term health surveillance project that collects health information on all residents of Olmsted County. They identified all women who had a first episode of venous thromboembolism during each trimester of pregnancy or during the postpartum period. They compared the incidence of such events in these women with the incidence in the general population of nonpregnant women in the same age range.

What did the researchers find?

During the 30-year period of observation, 50, 000 women in Olmsted County had gone through pregnancy. Of these women, 76 had deep vein thrombosis and 24 had pulmonary embolism during their pregnancies or in the 3-month postpartum period. The frequency of venous thromboembolism among these women was 4 times greater than among women in the nonpregnant population. In general, risk for venous thromboembolism increased with each successive trimester of pregnancy but was highest in the postpartum period. In fact, the overall incidence of venous thromboembolism was 4 times higher in the postpartum period than it was during pregnancy, and the incidence of pulmonary embolism was 15 times higher. During pregnancy, younger women had a greater risk of venous thromboembolism than older women; in the postpartum period, the risk was greater among older women. Over the course of the 30-year study, the incidence of venous thromboembolism remained constant during pregnancy but decreased by half among postpartum women.

What are the limitations of the study?

Because the ethnicity of the Olmsted County population is almost entirely white, the findings may be different for other ethnic groups.

What are the implications of the study?

In women with underlying risk for venous thromboembolism, the postpartum period requires special attention. Under certain circumstances, the patient's risk may justify the use of blood-thinning medication.