Comment: The quality of evidence is upgraded by large magnitude of effect.
A retrospective population-based registry study «Kamel H, Navi BB, Sriram N et al. Risk of a thromb...»1 used claims data on all discharges in California years 2005 – 2010 to find out a composite primary outcome of ischemic stroke, acute myocardial infarction, or venous thromboembolism in pregnant and postpartum women. Among the 1 687 930 women with a first recorded delivery, 1015 had a thrombotic event in the period of 1 year plus up to 24 weeks after delivery. The risk of primary thrombotic events was markedly higher within 3 weeks after delivery than in the same period 1 year later (odds ratio (OR 46.7, 95% CI 34.5-63.1) and within 4 to 6 weeks postpartum (OR 4.4, 95% CI 3.3-5.9). There was also a modest but significant increase in risk during weeks 7 to 12 after delivery as compared with the same period 1 year later (OR 2.2, 95% CI 1.5 to 3.1). Risks of thrombotic events were not significantly increased beyond the first 12 weeks after delivery.
Another registry study «Abdul Sultan A, Tata LJ, Grainge MJ et al. The inc...»2 involving a total of 1 117 691 women used primary care data from the Clinical Practice Research Datalink (CPRD), which incorporates linkages to secondary care contained within Hospital Episode Statistics (HES) in United Kingdom between 1997 and 2010. The absolute venous thromboembolism (VTE; deep vein thrombosis and pulmonary embolism) rate during the antepartum period and first 6 weeks postpartum were 99 (95%CI 85-116) and 468 (95%CI 391-561) per 100 000 person-years respectively. These were comparable to the pooled estimates from the meta-analysis (using studies after 2005) during the antepartum period (118/100 000 person-years) and early postpartum (424/100 000 person-years).
A population-based inception cohort study «Heit JA, Kobbervig CE, James AH et al. Trends in t...»3 estimated the relative and absolute risk for deep venous thrombosis and pulmonary embolism (VTE) during pregnancy and postpartum trends in incidence. Patients were women (98% white and of non-Hispanic ethnicity) with VTE first diagnosed between 1966 and 1995, including women with VTE during pregnancy or the postpartum period. The relative risk (standardized incidence ratio) for VTE 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). Over the 30-year study period, the incidence of venous thromboembolism during pregnancy remained relatively constant.
A study «Tepper NK, Boulet SL, Whiteman MK et al. Postpartu...»4 used deidentified health care claims information database across the United States to identify delivery hospitalizations among women aged 15-44 years during the years 2005-2011. ICD-9, Clinical Modification diagnosis and procedure codes were used to find VTE among women with recent delivery. The incidence proportion of postpartum venous thromboembolism was highest during the first 3 weeks after delivery, dropping from 9/10 000 during the first week to 1/10 000 at 4 weeks after delivery and decreasing steadily through the 12th week.
Another historical controlled national cohort study «Virkus RA, Løkkegaard EC, Bergholt T et al. Venous...»5 used The National Registry of Patients identified relevant diagnoses including Danish women 15 to 49 years old during years 1995 – 2005 to assess the relative risk of VTE. In total 819 751 pregnant women were included of whom 727 had a VTE. The absolute risk of VTE per 10 000 pregnancy-years increased from 4.1 (95% CI 3.2 to 5.2) during week 1 - 11 up to 59.0 (95% CI 46.1 to 76.4) in week 40 and decreased in the puerperal period from 60.0 (95% CI 47.2 to 76.4) during the first week after birth to 2.1 (95% CI 1.1 to 4.2) during week 9-12 postpartum. Compared with non-pregnant women, the incidence rate ratio rose from 1.5 (95% CI 1.1 to 1.9) in week 1 - 11, to 21.0 (95%CI 16.7 to 27.4) in week 40 and 21.5 (95% CI: 6.8 to 27.6) in the first week after delivery, declining to 3.8 (95% CI 2.5 to 5.8) 5 - 6 weeks after delivery.