Five Days of Simulated Weightlessness Changes How Women's Blood Clots

In 2020, a female astronaut aboard the International Space Station was found to have a blood clot in her jugular vein - discovered incidentally during a routine research ultrasound that was not even looking for one. The finding was surprising enough that NASA began including jugular vein scans in standard mission health checks. But the deeper question it raised - how does weightlessness affect blood clotting, and does it affect women differently from men? - had barely been studied.

A new study from Simon Fraser University, conducted in collaboration with the European Space Agency, begins to fill that gap. It is not a large study, and the conditions are simulated rather than actual spaceflight. But its findings are specific enough to matter.

Five days floating, without leaving Earth

The ESA-sponsored VIVALDI I study used a dry immersion setup: a water bath with a waterproof sheet that allows participants to float - experiencing the fluid redistribution of weightlessness - while staying dry. Eighteen healthy female volunteers spent five continuous days in the tank. This is one of the most effective ground-based methods for simulating the physiological effects of microgravity, particularly the shift of body fluids toward the upper body that happens when gravity no longer pulls blood toward the legs.

Researchers measured clotting responses using rotational thromboelastometry (ROTEM), a diagnostic method that tracks how blood clots form from initiation through final structure. They also analyzed participants' menstrual hormone levels, which turned out to have no significant effect on clotting outcomes.

Slower to start, faster to finish, harder to break down

The pattern that emerged was internally consistent but counterintuitive. Coagulation time - how long it took clotting to begin - was longer in the simulated microgravity environment. But once clotting initiated, it formed faster and produced clots that were stronger and more structurally stable than normal.

"In this microgravity environment, we found the female participants took longer for their blood to start clotting. But once that clotting began, it formed faster and was more stable, making it harder to break down," said Andrew Blaber, professor of biomedical physiology and kinesiology at SFU and senior author of the study.

Lead author Tiffany Stead was careful to note that this combination was not shown to be clinically dangerous after five days. But it raises concerns about longer missions - six months on the ISS, or the multi-year journey a Mars mission would require.

Why location changes everything

The danger of blood clots depends not just on their composition but on where they form. On Earth, gravity pulls blood toward the lower body, so deep vein thrombosis - the most common form of blood clot - typically develops in the legs. From there, it has a long distance to travel before reaching the lungs or heart, giving the body more time to dissolve it or doctors more time to treat it.

In microgravity, that geography changes. Blood redistributes toward the upper body and the head. Venous return slows in the neck. The 2020 ISS case is an example of what that redistribution can produce: a clot in the jugular vein, much closer to the lungs and heart than a leg clot would be.

"We've found that in space, blood clots are more likely to form in the jugular vein. From there, it doesn't have to travel far to reach lungs or heart, and trigger a serious medical event," Blaber said. "Space is not a place where you want these things to happen."

What comes next

Blaber's team is now analyzing comparable data from male participants in dry immersion studies, which will allow direct comparison between the two datasets and help identify whether the clotting changes seen in women are sex-specific or part of a broader physiological response to simulated weightlessness.

Space agencies are already taking precautionary steps. Jugular vein ultrasound scans are now part of regular health monitoring on ISS missions - driven in part by the 2020 discovery. The SFU-ESA findings suggest that monitoring alone may not be sufficient for longer missions, and that countermeasures - anticoagulant protocols, perhaps, or targeted exercise regimens - may eventually be needed.

"Now that they know it can happen, they're looking at it more frequently as part of the standard measures," Blaber said. The next step is understanding what to do about it.