When the human body evolves in microgravity, it is mishandled and its physiological balance is profoundly disturbed: weightlessness leads to muscle wasting and bone demineralization; this applies to both men and women. But in the case of the latter, they would be more conducive to developing thromboses (formation of blood clots) in conditions very different from those observed on Earth. This is a study published in March 2026 in the journal Acta Astronautica who has just documented this phenomenon, with supporting experimental data that had never been collected until now.
Microgravity: women’s blood systems under pressure
To reproduce the effects of weightlessness without leaving the ground, the team of the Simon Fraser University (SFU) used dry immersion. A fairly grueling protocol, since the 18 participants in the study remained floating for five days in a thermostatically controlled bath, isolated from the water by a waterproof tarpaulin. This device neutralizes the pressure on the body’s pressure points and thus simulates the redistribution of fluids: in the absence of gravity, the blood no longer goes down the legs and thus flows back towards the trunk and the head.
The researchers then analyzed the kinetics of coagulation, that is to say the speed and way in which the blood solidifies to seal a breach. To do this, they used the rotational thromboelastometry (RED), a real-time analysis technique that measures the viscoelasticity of blood during the formation of a clot.
After five days, all the volunteers showed signs of hypercoagulability: their blood had clotted abnormally. The data shows that if coagulation starts more slowly, once initiated, clot formation is much faster than normal.
The clots themselves are different from clots that can form on Earth: they are much denser, and resist more when the body naturally tries to dissolve them (fibrinolysis phenomenon).
In addition to this accelerated formation, clots tend to appear in areas considered risky in the blood network. « In space, clots are more likely to appear in the jugular vein. From there, they can quickly reach the lungs or heart and trigger a serious event », Specifies researcher Richard Blaber, co-author of the study.
A problem for long-term missions
The researchers, however, want to be reassuring: over a five-day immersion, no problems were identified among the women tested for the occasion. But if we look further, notably the lunar base projects of the Artemis program, or worse, the first manned flights to Mars, prolonged exposure to weightlessness becomes a more significant risk factor for a female crew member. By spending entire months without benefiting from the effects of Earth’s gravity, the probability of seeing this type of clot appear increases statistically.
We won’t draw a picture for you: millions of kilometers from Earth, no emergency evacuation is possible. A migrating clot can cause a pulmonary embolism or a stroke and without heavy hospital infrastructure to carry out thrombolysis (the chemical dissolution of the clot), a whole crew can be threatened.
This study also provides us with crucial precision for monitoring these missions: menstrual cycles do not seem to influence this coagulation. The risk of hypercoagulability is a physical response of the female vascular system, which therefore has nothing to do with any hormonal particularity. Fortunately, space agencies did not wait for this work to generalize jugular ultrasoundsmandatory for all crews staying on board the ISS. Surveillance inherited from the accidental discovery of a clot at the American astronaut Serena Auñón-Chancellor in 2019. Let us now hope that they will be used to establish a stricter monitoring protocol imposed for missions including mixed crews.
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