What mouse embryos can tell us about the future of pregnancies in space

Pregnant space travelers are ‘not a question of, if but when.'
The silver structure of the International Space Station against the black backdrop of space.
Mouse embryos and sperm seem to handle microgravity on the ISS without major problems. NASA

We may be a spacefaring species, but only a tiny vanguard have actually explored beyond Earth’s atmosphere. Fewer than 700 people have flown in space, and the vast majority of those have been white men with a military background, screened for health and skills. But astronauts’ demographics are rapidly changing. Commercial space companies have sent space tourists on suborbital and orbital space flights, such as the all-civilian men and women of the SpaceX Inspiration 4 mission. Multiple companies plan to launch private space stations after the International Space Station is retired. NASA, meanwhile, has promised that a woman will be the first astronaut to set foot on the moon again when the Artemis III mission lands on the lunar south pole. And, in subsequent missions, the space agency plans to build long-term habitats on the moon. 

With more humans headed to space than ever, there’s an opportunity for all kinds of medical scenarios to crop up—especially those that haven’t occurred among the previous cadre of professional astronauts. Space travelers could have heart attacks, suffer traumatic injuries, or, as a result of one of the most human of activities, become pregnant.  

“It’s not a question of if, but when,” says physician Emmanuel Urquieta, the chief medical officer at the Translational Research Institute for Space Health, or TRISH, at Baylor College of Medicine. The problem, he says, is that the small sample of humans who have flown in space provides very little knowledge of how average body will respond to long-term flights. That goes double for conception, pregnancy, and the delivery of a baby, where there is no human spaceflight data at all. Numerous factors such as low gravity and high radiation are thought to pose risks to the healthy development of a fetus or the birth of a child. 

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These aren’t simply academic gaps to fill. “If we’re planning to develop habitation capabilities, and off-Earth colonies on the moon and Mars, this is something that will absolutely need to be solved,” Urquieta says. 

Scientists have just completed a very basic start. One new study published in the journal iScience by researchers at the Japan Aerospace Space Agency, JAXA, and the Japan Aerospace Space Agency may provide optimistic, if provisional, evidence that pregnancy in space is possible. At least, for mice. 

In August 2021, the research team sent frozen mouse embryos to the ISS, where, once thawed, they developed in the space station’s microgravity environment. After the embryos were returned to Earth about a month later, the study authors found that the small clusters of cells grew as normal. Each embryo formed two cellular structures known as a blastocyst and an inner cell mass; if allowed to develop further, those would go on to become the placenta and fetus, respectively. The researchers had worried that without gravity, the inner cell mass would not be able to coalesce in one space within the blastocyst. 

The research is another piece of evidence that mammalian fertility works in the conditions of spaceflight. Past experiments have shown that mouse sperm flown in space produced viable offspring when returned to Earth. Although there is a large gap between this early stage of embryonic development and birth of a healthy animal, the study team plans to conduct such a test in the future. 

And, of course, this finding was in mice. Urquieta cautions that it’s hard to tell how mouse results translate to human health even when experiments take place within Earth’s normal gravity. “A general challenge in human spaceflight is that a lot of the research that we have is from animal models,” he says. ”How much of those results could be extrapolated to humans still remains a question.”

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Even if a fetus can develop in space, several key challenges must be addressed for a human mother off Earth. The first is nutrition, because pregnant people need sufficient protein and levels of folic acid to support a healthy fetal development. “Providing macro and micronutrients in spaceflight is going to be challenging,” Urquieta says, in a space station environment where fresh foods are in short supply. Lunar or Mars colonies probably won’t even have the luxury of regular deliveries from Earth. 

Then there’s radiation. Not all the mouse embryos developed successfully in the new study, and the researchers suspect that radiation could be the cause. “We know that radiation is very damaging in general to cells, and especially during the first three or four weeks of pregnancy,” Urquieta says. The ISS orbits low enough that it’s shielded by Earth’s magnetosphere, he says, but on the moon or a trip to Mars, the full brunt of galactic cosmic radiation could become a problem. 

Being pregnant on Earth isn’t a garden stroll, either, and it would probably be even less comfortable in space. Certain well-documented physiological changes in microgravity include shifting bodily fluids in for instance, with blood collecting in the head and overall blood volume decreasing. “There’s also space motion sickness, nausea, and vomiting. We know that that is also something common in pregnancy,” Urquieta says. “It would definitely exacerbate the non-pleasant symptoms.” 

Ultimately, he says, he researchers who study reproduction in space need to think about crawling before they walk—finding general solutions for astronaut radiation exposure and nutritional needs at lunar bases before tackling the specific requirements of pregnant astronauts. But given the likely inevitability of human space pregnancies, he says, “I think it’s important we start the conversations, and also increase awareness that this is going to be a very, very complex and challenging issue to solve.”