This calculator is designed to predict the risk of a VTE (blood clot) in the 6-weeks following childbirth, to aid healthcare professional decisions as to which women would receive thromboprophylaxis with low molecular weight heparin for 10 days duration. These women are considered to be at an intermediate risk of a postpartum VTE according to the green top guidelines issued by the Royal College of Obstetricians and Gynaecologists (RCOG). Any woman who has had a blood clot previously would be considered high risk by the RCOG and receive thromboprophylaxis for at least 6 weeks, regardless of other risk factors so this tool should not be used for these women. Some women who have been diagnosed with a hereditary thrombophilia would also be considered high risk. As we did not have data available to us on inherited thrombophilia when developing this model, this tool should avoided in this instance as well.

This calculator is primarily for use by health professionals but could be used by pregnant women who have some knowledge of blood clots and an appreciation that these are a serious but rare complication of pregnancy and childbirth.

Health Professionals: This calculator relies on all information being entered completely, so for instance if there is no mention of varicose veins before delivery in the woman’s medical notes then it is assumed that this risk factor is absent. For height and weight of the mother and birthweight of the child, you have the option of entering these data using your preferred units of measurement.

Pregnant women: If you are concerned about the result obtained and worried you may be at risk of a blood clot, then you should discuss this result with a relevant health professional before making any changes to lifestyle or medications.

1 Sultan AA, West J, Tata LJ, Fleming KM, Nelson-Piercy C, Grainge MJ. Risk of first venous thromboembolism in and around pregnancy: A population-based cohort study. British Journal of Haematology 2012; 156: 366–73.

2 Kamel H, Navi BB, Sriram N, Hovsepian DA, Devereux RB, Elkind MSV. Risk of a Thrombotic Event after the 6-Week Postpartum Period. New England Journal of Medicine 2014; 370: 1307–15.

3 Royal College of Obstetricians and Gynecologists. Reducing the Risk of Venous Thromboembolism during Pregnancy and the Puerperium. Green-top Guideline No 37a 2015.

4 Sultan AA, West J, Grainge MJ, Riley RD, Tata LJ, Stephansson O, Fleming KM, Nelson-Piercy C, Ludvigsson JF. Development and validation of risk prediction model for venous thromboembolism in postpartum women: Multinational cohort study. BMJ 2016 DOI:10.1136/bmj.i6253.

This calculator was developed by a team of researchers in the Division of Epidemiology and Public Health at the University of Nottingham to predict the risk of a venous thromboembolism (VTE, deep vein thrombosis or pulmonary embolism), or blood clot as it is commonly known, in the six-weeks following childbirth. The team have collaborated previously on several studies predicting which women were most likely to get VTE around the time of pregnancy and when they were most at risk using data from electronic health records. Some of this work was carried out as part of a PhD by Dr Alyshah Abdul Sultan, who was the primary developer of this risk score. The calculator was developed to predict postpartum VTE specifically, because the risk of a blood clot is highest following childbirth, declining rapidly after a peak in the first 3 weeks following childbirth (1,2). Current guidelines in the UK issued by the Royal College of Obstetricians and Gynaecologists, advise that women considered to be at intermediate risk of VTE (based on commonly occurring risk factors) should receive thromboprophylaxis for 10 days (3).

The present model was developed using data from women in England who gave birth between 1997 and 2014 and whose data was recorded in the Clinical Practice Research Datalink (CPRD, covering 6% of the population of the UK)(4). The model was then externally validated using linked healthcare data from Swedish women who gave birth between 2005 and 2011. We found that our risk model more accurately predicted which women developed VTE (higher sensitivity and positive predictive value) than the algorithm used by the RCOG in both the dataset the model was developed in and the independent Swedish dataset. This model is subsequently undergoing further external validation in a completely separate database of electronic medical records from the UK (QResearch). Whilst the original model was published in November 2016 in the British Medical Journal our tool was named the Maternity Clot Risk in December 2018.

If you have any queries relating to this calculator, please contact either Matthew Grainge or Yue Huang.