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Body Mass Index, Surgery, and Risk of Venous Thromboembolism in Middle-Aged Women: A Cohort Study

Parkin L, Sweetland S, Balkwill A, Green J, Reeves G, Beral V, for the Million Women Study collaborators.

Circulation 2012; 125 :1897-1904.


Obesity and surgery are known risk factors for venous thromboembolism (VTE), but there is limited information about the independent effects of obesity on the incidence of post-operative VTE. We linked questionnaire data from the Million Women Study with hospital admission and death records to examine the risk of VTE in relation to body mass index (BMI) both in the absence of surgery and in the first 12 weeks following an operation.


1,170,495 women (mean age 56.1 years) recruited in 1996-2001 through the NHS Breast Screening Programme in England and Scotland were followed for an average of six years, during which time 6,438 were admitted to hospital or died from VTE. The adjusted relative risks of VTE increased progressively with increasing BMI and women with a BMI ≥35 kg/m(2) were more than three times as likely to develop VTE than those with a BMI 22.5-24.9 (RR 3.45 [95% CI 3.09 to 3.86]). Overweight and obese women were also more likely than lean women to be admitted for surgery and to develop post-operative VTE. During a 12 week period without surgery, the incidence rates of VTE per 1000 women with a BMI <25 and ≥25 were 0.10 (0.09 to 0.10) and 0.19 (0.18 to 0.20); the corresponding rates in the 12 weeks following day and inpatient surgery were, respectively, about four and forty times higher.


VTE risk increases with increasing BMI and the associated excess risk is much greater following surgery than without surgery.