Maternity Clot Risk

The risk of a blood clot is highest following childbirth, declining rapidly after a peak in the first 3 weeks following childbirth.^12,3 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.⁴ The current algorithm used by the RCOG uses combinations of commonly occurring risk factors (e.g. caesarean delivery, pre-existing medical conditions and obesity) to predict which women will receive thromboprophylaxis with low molecular weight heparin for 10 days following childbirth. Our calculator was developed to formalise this by deriving an actual risk of postpartum VTE (expressed as a probability).

The team who developed this model 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. A fair body 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. He was supported other members Division of Epidemiology and Public Health at Nottingham University including Matthew Grainge, Laila Tata and Joe West. The work was carried out in collaboration with St. Guys and Thomas Hospital in London and the Karolinska Institute in Stockholm. 

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).⁵ 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 formally named the Maternity Clot Risk in December 2018.

This calculator is designed primarily for use by health professionals who would be involved in decisions surrounding VTE prophylaxis around the time of childbirth. However, it has been developed in such a way that it could be used by pregnant women themselves. This calculator is designed to predict the risk of a VTE 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 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.

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. Br J Haematol 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. N Engl J Med 2014; 370: 1307–15.
3. Tepper NK, Boulet SL, Whiteman MK, et al. Postpartum venous thromboembolism: Incidence and risk factors. Obstet Gynecol 2014; 123: 987–96.
4. Royal College of Obstetricians and Gynecologists. Reducing the Risk of Venous Thromboembolism during Pregnancy and the Puerperium. Green-top Guidel No 37a 2015.
5. Sultan AA, West J, Grainge MJ, et al. Development and validation of risk prediction model for venous thromboembolism in postpartum women: Multinational cohort study. BMJ 2016 DOI:10.1136/bmj.i6253.