Risk of first venous thromboembolism in pregnant women in hospital: population based cohort study from England

Objective To examine the potential for preventing venous thromboembolism during and after antepartum hospital admissions in pregnant women. Design Cohort study using linked primary (Clinical Practice Research Datalink) and secondary (Hospital Episode Statistics) care records. Setting Primary and secondary care centres, England. Participants 206 785 women aged 15-44 who had one or more pregnancies from 1997 up to 2010. Main outcome measure Risk of first venous thromboembolism in pregnant women admitted to hospital for one or more days for reasons other than delivery or venous thromboembolism. Risk was assessed by calculating the absolute rate of venous thromboembolism and comparing these rates with those observed during follow-up time not associated with hospital admission using a Poisson regression model to estimate incidence rate ratios. Results Admission to hospital in pregnancy was associated with an increased risk of venous thromboembolism (absolute rate 1752/100 000 person years; incidence rate ratio 17.5, 95% confidence interval 7.69 to 40.0) compared with time outside hospital. The rate of venous thromboembolism was also high during the 28 days after discharge (absolute rate 676; 6.27, 3.74 to 10.5). The rate during and after admission combined was highest in the third trimester (961; 5.57, 3.32 to 9.34) and in those aged >= 35 years (1756; 21.7, 9.62 to 49.0). While the absolute rate in the combined period was highest for those with three or more days in hospital (1511; 12.2, 6.65 to 22.7), there was also a fourfold increase (558; 4.05, 2.23 to 7.38) in the risk of venous thromboembolism for those admitted to hospital for less than three days. Conclusion The overall risk of first venous thromboembolism in pregnant women increased during admissions to hospital not related to delivery, and remained significantly higher in the 28 days after discharge. During these periods need for thromboprophylaxis should receive careful consideration.