Spending just a few days in a simulated microgravity environment can subtly change the way a woman’s blood clots, according to researchers at Simon Fraser University, raising big questions about health monitoring protocols for astronauts who may spend more than six months in orbit.
First reported in 2020, an unexpected blood clot was detected in the jugular vein of a female astronaut on a mission to the International Space Station. Space health research has traditionally been dominated by men, but as the number of female astronauts increases, a new study from SFU and the European Space Agency investigated how microgravity affects blood clotting, especially in women.
Main findings
- Eighteen female participants experienced five days of continuous simulated microgravity in the VIVALDI I dry immersion study sponsored by the European Space Agency (ESA).
- The clotting time (the time it takes for a blood clot to start forming) has increased.
- Once started, blood clots formed faster.
- Once formed, the strength and stability of the clot was greater
- The findings indicate that spending just five days in a simulated microgravity environment was not of clinical concern, but further research is needed to assess the potential risks of longer spaceflights.
We know that men and women may clot differently depending on their age on Earth, but we have little information about whether it’s different in space. They found that in this microgravity environment, it took longer for female participants’ blood to start clotting. But once it began to coagulate, it formed faster and became more stable, making it harder to break down. ”
Andrew Blaver, professor of biomedical physiology and kinesiology and senior author of the study
Tiffany Stead, lead author of the study, said this combination (slower onset, faster formation, stronger clots) has not been shown to be inherently dangerous in the short term. But for astronauts, there are concerns about how and where dangerous blood clots can form in the body while in space, where emergency medical care is not available.
Published in acta astronauticaThe study examined real-time clotting reactions in 18 healthy female participants over five days in a dry immersion tank (a specially designed water tank with a tarpaulin to keep participants wet) that simulates weightlessness while floating.
The study used a diagnostic method called rotational thromboelastometry (ROTEM) to measure how clotting begins and progresses in the body. The participants’ blood was also analyzed for menstrual hormones, which were found to have no effect on blood clotting.
Why spaceflight changes where and how dangerous blood clots form
If left untreated, the blood clot can break off and travel through the bloodstream. If they reach the lungs, heart, or brain, they can cause pulmonary embolisms, heart attacks, and strokes.
Because of Earth’s gravity, blood clots most commonly form in the legs, allowing the body to break them down on their own or buy time for medical attention before they become life-threatening.
But without gravity, blood pools in the head and sometimes reverses direction, creating a condition where blood clots are more likely to form, Braver said.
“We found that in space, blood clots are more likely to form in the jugular vein, and from there they don’t have to travel far to reach the lungs or heart and cause serious medical problems,” he says. “Space is not a place where you want these things to happen.”
Blaber’s team is currently analyzing comparable men’s dry soak data, which will help guide future medical monitoring and countermeasures needed during spaceflight.
Space agencies are already paying close attention. Astronauts now routinely perform ultrasound scans of their jugular veins during missions, the very technique that happened to reveal the first blood clot in 2020.
“Now that we know it can happen, we’re testing more frequently as part of our standard precautions,” Braver said.
Blaver and his team at SFU’s Institute of Aerospace Physiology regularly collaborate with the Canadian Space Agency and the International Space Agency to study the effects of space on astronauts.
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Reference magazines:
Stead, T.E. Others. (2026). Changes in blood coagulation ability in women exposed to dry immersion: Investigation of the mechanism of development of venous thromboembolism under microgravity. acta astronautica. DOI: 10.1016/j.actastro.2025.11.065. https://www.sciencedirect.com/science/article/pii/S0094576525008379?via%3Dihub
