Trends in the incidence of venous thromboembolism during pregnancy or postpartum: a 30-year population-based study.

BACKGROUND: The risk for venous thromboembolism during pregnancy or postpartum is uncertain. OBJECTIVES: To estimate the relative and absolute risk for deep venous thrombosis and pulmonary embolism during pregnancy and postpartum and to describe trends in incidence. DESIGN: Population-based inception cohort study using the resources of the Rochester Epidemiology Project. SETTING: Olmsted County, Minnesota. PATIENTS: Women with deep venous thrombosis or pulmonary embolism first diagnosed between 1966 and 1995, including women with venous thromboembolism during pregnancy or the postpartum period (defined as delivery of a newborn no more than 3 months before the deep venous thrombosis or pulmonary embolism event date, including delivery of a stillborn infant after the first trimester). MEASUREMENTS: The authors obtained yearly counts of live births in Olmsted County between 1966 and 1995 from the Minnesota Department of Health. RESULTS: The relative risk (standardized incidence ratio) for venous thromboembolism among pregnant or postpartum women was 4.29 (95% CI, 3.49 to 5.22;P < 0.001), and the overall incidence of venous thromboembolism (absolute risk) was 199.7 per 100,000 woman-years. The annual incidence was 5 times higher among postpartum women than pregnant women (511.2 vs. 95.8 per 100,000), and the incidence of deep venous thrombosis was 3 times higher than that of pulmonary embolism (151.8 vs. 47.9 per 100,000). Pulmonary embolism was relatively uncommon during pregnancy versus the postpartum period (10.6 vs. 159.7 per 100,000). Over the 30-year study period, the incidence of venous thromboembolism during pregnancy remained relatively constant whereas the postpartum incidence of pulmonary embolism decreased more than 2-fold. LIMITATIONS: Because the Olmsted County population was 98% white and of non-Hispanic ethnicity, the results may not be generalizable to other ethnicities. CONCLUSIONS: Among pregnant women, the highest risk period for venous thromboembolism and pulmonary embolism in particular is during the postpartum period. Any prophylaxis against these events should be particularly targeted to postpartum women. Although the incidence of pulmonary embolism has decreased over time, the incidence of deep venous thrombosis remains unchanged, indicating the need to better identify pregnant women at increased risk.

FXIII polymorphisms, fibrin clot structure and thrombotic risk.

Fibrin clot structure is highly dependent on factor XIII activity. Activated FXIII catalyzes the formation of the peptide bonds between the gamma and alpha chains in noncovalently bound fibrin polymers and incorporates various adhesive and antifibrinolytic proteins into the final fibrin clot. In the absence of activated FXIII, clots are unstable and susceptible to fibrinolysis. Several studies have examined the effects of FXIII polymorphisms on final fibrin clot structure and clinical thrombotic risk. The Val34Leu FXIII polymorphism is associated with increased activation by thrombin. In the presence of saturating thrombin concentrations, however, FXIIIa specific enzyme activity is not affected by genetic polymorphisms. Fibrin clots formed in the presence of the FXIII 34Leu polymorphisms do tend to be thinner and less porous, however. The effects of prothrombin concentrations on clot structure have suggested that thinner clots are more resistant to fibrinolysis and associated with increased thrombotic risk. Most clinical studies of 34Leu FXIII carriers, however, have demonstrated a lower incidence of both venous and arterial thrombosis in carriers of the mutant allele compared to Val/Val carriers. One recent study has suggested that the interactions between FXIII phenotype and plasma fibrinogen concentrations significantly influence clinical thrombotic risk.