Thrombosis Australia

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Our Thrombosis Australia Advisory Panel consists of eminent Australian healthcare professionals.

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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.

Oral contraception

Using oral contraceptives increases the risk for venous thromboembolism by a factor of 3 to 5 and women who have genetic or other health conditions associated with thrombosis will have an even higher risk. These conditions include thrombophilia and Factor V Leiden.

Research published in 2022 found that the use of progestin-only contraception was not associated with an increased risk of venous thromboembolism.

A 2023 study investigated how long it might take to normalise the oestrogen-related thrombotic biomarkers after stopping combined hormonal contraceptives (CHC). The research found a rapid decrease in the biomarkers, with 80% of the reduction by 2 weeks.

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 probability 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.

Superficial vein thrombosis during pregnancy

Superficial vein thrombosis (SVT) or superficial thrombophlebitis is an inflammatory process which causes a clot to form and block a vein, near the surface of the skin, typically in the legs. Recent research investigated the incidence of SVT during and after pregnancy, and the risk of developing a venous thromboembolism (VTE). The study found that throughout the pregnancy and during the post-partum period, the risk of SVT was low. However, if an SVT did occur, the risk of developing VTE during the same pregnancy was high. These findings may be relevant for physicians and women during pregnancy to make decisions about anticoagulant management of pregnancy-related SVT.

PCOS and thrombosis

PCOS is a hormonal disorder and is one of the most common endocrine system and reproductive disorders among women, with the global prevalence in 2020 being 5-20%. It affects not only the reproductive system, but also the metabolic and cardiovascular systems. In 2020, research found that women with PCOS have a 1.5-2.0-fold increased risk of venous thromboembolism (VTE) compared to women without the condition. Please visit our 'information page' to read more about PCOS and thrombosis.

von Willebrand Disease

Von Willebrand disease (VWD) is an inherited bleeding disorder where the blood does not clot properly and people with the condition have low levels or underperforming von Willebrand factor (VWF). VWF is an important protein which is involved in the blood clotting process. Women are more likely to experience symptoms due to the bleeding which occurs during menstruation, pregnancy, and post-partum.

VWF works with the clotting protein Factor VIII. VWF and Factor VIII increase during pregnancy, and the goal of these changes is to facilitate haemostasis after childbirth. Haemostasis is the body’s response to stop bleeding and involves complex interactions between cells, platelets and coagulation proteins such as Factor VIII.

Excess gynaecological bleeding is increased in women with VWD, with post-partum haemorrhage increasing 1.5 times and a 5-fold risk of blood transfusion and a higher risk of death by 10-fold. Research literature has shown that risk of bleeding at parturition (act of giving birth) in women with VWD ranges from 15%-60%, with an increased risk of post-partum bleeding up to 6-weeks following delivery, 30% compared to 2% among women without the condition. Management of pregnant women with VWD is quite complex and research does recommend antenatal assessment of VWF levels, and third trimester levels should be monitored. Click HERE for additional information.


  • 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
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