Key Points• Antepartum, we found that established risk factors only had a modest effect on rates of VTE.• Postpartum, we found that among other factors, women with stillbirth or preterm birth had high rates of VTE.Knowledge of the absolute risk (AR) for venous thromboembolism (VTE) in women around pregnancy and how potential risk factors modify this risk is crucial in identifying women who would benefit most from thromboprophylaxis. We examined a large primary care database containing 376 154 pregnancies ending in live birth or stillbirth from women aged 15 to 44 years between 1995 and 2009 and assessed the effect of risk factors on the incidence of antepartum and postpartum VTE in terms of ARs and incidence rate ratios (IRR), using Poisson regression. During antepartum, varicose veins, inflammatory bowel disease (IBD), urinary tract infection, and preexisting diabetes were associated with an increased risk for VTE (ARs, ‡139/100 000 person-years; IRRs, ‡1.8/100 000 personyears). Postpartum, the strongest risk factor was stillbirth (AR, 2444/100 000 personyears; IRR, 6.2/100 000 person-years), followed by medical comorbidities (including varicose veins, IBD, or cardiac disease), a body mass index (BMI) of 30 kg/m 2 or higher, obstetric hemorrhage, preterm delivery, and caesarean section (ARs, ‡637/100 000 person-years; IRRs, ‡1.9/100 000 person-years). Our findings suggest that VTE risk varies modestly by recognized factors during antepartum; however, women with stillbirths, preterm births, obstetric hemorrhage, caesarean section delivery, medical comorbidities, or a BMI of 30 kg/m 2 or higher are at much higher risk for VTE after delivery. These risk factors should receive careful consideration when assessing the potential need for thromboprophylaxis during the postpartum period. (Blood. 2013;121(19):3953-3961)

Summary Knowledge of the absolute and relative risk of venous thromboembolism (VTE) in and around pregnancy would be crucial in identifying when to commence and cease thromboprophylaxis in women who would benefit from such intervention. We addressed this hypothesis using a large prospective primary care database from the United Kingdom, containing details on 972 683 women aged 15–44 years between 1987 and 2004. Risks of a first VTE during antepartum, postpartum and outside of pregnancy were compared using Poisson regression. The rate of VTE during the third trimester antepartum was six times higher than time outside pregnancy [Incidence Rate Ratio (IRR) = 6·1; 95% confidence interval, 4·7–7·9]. In contrast, both the first (IRR = 1·6) and second (IRR = 2·1) trimesters conferred little increase in risk. The first 6 weeks postpartum was associated with a 22‐fold increase in risk, with the peak occurring in the first 3 weeks. Increased age was found to be associated with VTE during postpartum and outside of pregnancy, but not during antepartum. Our findings of a notably raised risk of VTE persisting for 3 weeks postpartum and of a raised antepartum risk constrained to the third trimester have implications for modifying the current recommendations for VTE prophylaxis in pregnancy and the puerperium.

Objective To develop and validate a risk prediction model for venous thromboembolism in the first six weeks after delivery (early postpartum).Design Cohort study.Setting Records from England based Clinical Practice Research Datalink (CPRD) linked to Hospital Episode Statistics (HES) and data from Sweden based registry.Participants All pregnant women registered with CPRD-HES linked data between 1997 and 2014 and Swedish medical birth registry between 2005 and 2011 with postpartum follow-up.Main outcome measure Multivariable logistic regression analysis was used to develop a risk prediction model for postpartum venous thromboembolism based on the English data, which was externally validated in the Swedish data.Results 433 353 deliveries were identified in the English cohort and 662 387 in the Swedish cohort. The absolute rate of venous thromboembolism was 7.2 per 10 000 deliveries in the English cohort and 7.9 per 10 000 in the Swedish cohort. Emergency caesarean delivery, stillbirth, varicose veins, pre-eclampsia/eclampsia, postpartum infection, and comorbidities were the strongest predictors of venous thromboembolism in the final multivariable model. Discrimination of the model was similar in both cohorts, with a C statistic above 0.70, with excellent calibration of observed and predicted risks. The model identified more venous thromboembolism events than the existing national English (sensitivity 68% v 63%) and Swedish guidelines (30% v 21%) at similar thresholds.Conclusion A new prediction model that quantifies absolute risk of postpartum venous thromboembolism has been developed and externally validated. It is based on clinical variables that are available in many developed countries at the point of delivery and could serve as the basis for real time decisions on obstetric thromboprophylaxis.