Thrombosis Australia

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In 2018 the Australian Commission on Safety and Quality in Health Care found that each year 30,000 Australians develop blood clots at a cost of $1.72 billion to the Australian health system. There are several factors which increase the risk of developing blood clots, including obesity, immobility, smoking, family history and many more. However, women are at a higher risk if using oestrogen-containing contraceptives or hormone replacement therapy (HRT) due to the increased levels of oestrogen in the body. Pregnancy, childbirth, and the postpartum period (approx. 6 weeks) are also risk factors.

Treatment options

Oral anticoagulants (OAC) are often the treatment option to reduce the risk of thrombotic events; however, they can also increase bleeding. Finding a balance between potentially escalating menstrual bleeding and that of thrombosis risk can be difficult. Abnormal or excessive bleeding affects one-third of menstruating women and this increases to approximately 70% for those on OACs.

The common duration of OAC treatment is approximately 3 months, however an individualised assessment is recommended.

Post-menopausal period and HRT

For post-menopausal women using HRT, blood clots are a possible complication, especially with oral oestrogen HRT. An analysis of the types of HRT used found that non-oral HRT was not associated with increased probably of thrombosis and that transdermal (via the skin) HRT provides the lowest risk.


Pregnant women are 5-10 times more likely to experience a blood clot compared with non-pregnant women of similar age. Women who are already on anticoagulants and are planning to become pregnant, the type of OAC is important.

Talk to your doctor for the best option recommended for you.

For further information please visit our website pregnancy and thrombosis

Risk of recurring venous thromboembolism (VTE)

Research published in April 2022 investigated the risk of VTE recurring during pregnancy after a previously diagnosed event. The study followed 189 women aged 15-49 over a 20 year (2000-2020) period.

A VTE occurred in 17 women: five in the first trimester, none in the second, and one in the third; 11 occurred postpartum. The majority of women (12) had a deep vein thrombosis (DVT) and five had pulmonary embolism (PE) ± DVT. One of the main findings from the study was that obesity or a prepregnancy BMI ≥30 was a risk factor for recurring VTE. Of the 17 pregnant women with VTE events, 11 were either obese (eight) or overweight (3xBMI 25-30).

As this is only a very small study, and a high BMI is only one parameter found to be risk factor, it still presents valuable information for further research. There are current larger studies underway to affect clinical practice and guidelines in obstetrics and thromboprophylaxis in pregnant women with previous VTE.

Pregnancy, COVID-19 vaccination and thrombosis

A review published in December 2021 looked at the effects or consequences of mRNA vaccination (Pfizer and Moderna vaccines). The study found no obvious differences on the frequency of thrombosis between vaccinated and unvaccinated pregnant women. In the last 3-4 months additional research also confirm that there is not an increased risk of venous thromboembolism or pulmonary embolism after vaccination during pregnancy.

The other COVID-19 vaccines (Oxford/AstraZeneca and Johsnon & Johnson), are not a mRNA vaccine but an adenovirus-based vaccine. To determine if adenovirus-based vaccines had an effect on coagulopathy or pregnancy, a systematic review was conducted containing research between 1966 and 2021. From 28 studies containing 1731 pregnant women, no coagulopathy events were reported.

Early diagnosis of VTE in pregnancy

As some symptoms of DVT can mimic pregnancy associated indications such as lower leg swelling, and pelvic and back pain, a research review found it is imperative to diagnose DVT early. DVT and PE are major factors of maternal mortality, therefore a detailed assessment can establish a risk profile that would guide clinical decisions. Timely and accurate diagnosis of VTE is essential for its management. 


de Moreuil, C. et al. (2022). Risk factors for recurrence during a pregnancy following a first venous thromboembolism: A French observational study. J Thromb Haemost. 2022;20:909–918

Dimitrios Rafail Kalaitzopoulos, D.R. et al., (2022). Management of venous thromboembolism in pregnancy. Thrombosis Research.

Houghton D.E., Wysokinski W., Casanegra A.I., et al. (2022). Risk of venous thromboembolism after COVID-19 vaccination. J Thromb Haemost. doi:10.1111/jth.15725

Leik, N., Ahmedy, F., Guad, R. M., & Baharuddin, D. (2021). Covid-19 vaccine and its consequences in pregnancy: Brief review. Annals of medicine and surgery, 72.

Micaily, I. & Bannow, B.T.S. (2021). VTE and anticoagulation in menstruating women. Thrombosis Update.

Pischel, L., Patel, K.M., Goshua, G., Omer, S.B. (2022). Adenovirus-Based Vaccines and Thrombosis in Pregnancy: A Systematic Review and Meta-analysis, Clinical Infectious Diseases.

Prasad, S. et al. (2022). Systematic review and meta-analysis of the effectiveness and perinatal outcomes of COVID-19 vaccination in pregnancy. Nature Communications.

Simard,C. et al., (2022). Management of anticoagulation in pregnant women with venous thromboembolism: An international survey of clinical practice. Thrombosis Research.