{♫Intro♫}.

Will we ever live in space long-term? Prominent scientists have argued that the future of our species could lie off-world.

But there's one thing we'd need to make that work: babies. It's not just about parenting — surely our intrepid astronauts would be up to that challenge. It's that some of them would eventually have to get pregnant in the first place.

As wild as it sounds, researchers are already trying to figure out if people can make space babies — but so far, the results don't look great. One major hurdle is the lack of gravity. That includes near-weightlessness, or microgravity, as well as places with less gravity than our home planet — like Mars, where gravity is less than half of what we're used to.

Most experiments that have tried to shed light on this have been done in real or simulated microgravity, so that's what we'll focus on here. But so far, scientists think that even Martian gravity would pose problems for conception and gestation. Overall, the story starts out okay: Menstruation in space seems to work normally, at least for short trips — astronauts often choose to skip their periods for longer missions.

So although we don't know for sure, that suggests that ovulation would also happen normally. Meanwhile, sperm seem to fare, well, kind of okay in microgravity. Like, in 1979, researchers flew live rats on a research satellite for eighteen days.

And when they got home, they successfully mated with ovulating rats. Except not all of their offspring were totally healthy — the growth and development of babies fertilized by mature sperm that were exposed to zero-g effects lagged behind control litters. The nice thing is, we might be able to get around this by using space sperm banks instead.

FST01 In research presented in 2019, scientists in Barcelona reported some preliminary evidence that frozen human sperm can survive short bursts of microgravity with no ill effects — though their evidence came from specialized aircraft, not from space. Still, it's a start. Now, having healthy cells is one thing, but to get a fetus, an egg needs to be fertilized.

And there is evidence that sperm can fertilize eggs in microgravity. One 2009 study used a spinning machine to simulate microgravity, and successfully carried out in vitro fertilization using mouse sperm and eggs. But even then, fertilization is still a step or three away from pregnancy.

Pregnancy isn't official until an embryo can implant into its host's uterine wall. For that, you need the formation of the blastocyst — a hollow, fluid-filled ball of cells that forms within a few days of fertilization. The layer of outer cells is called the trophoblast, which helps the embryo to burrow into the uterine wall and form the placenta.

There's also a clump of inner cells called the embryoblast, which gives rise to the fetus itself. And it seems like embryos might be able to do all that in space. Although it hasn't been reported in a peer-reviewed journal, an experiment containing mouse embryos that flew on China's first microgravity satellite saw some of them form blastocysts.

Which is promising, but we'll need more studies to confirm. But even if blastocysts can form in space, it's unclear whether they could implant and keep growing. In that same 1979 mission we mentioned, male and female rats were also sent into orbit and allowed to… mingle.

Some of them did get pregnant, but none of them gave birth — the researchers believed that the implanted embryos died and were absorbed back into the rats' bodies. Scientists are trying to figure out what exactly it is about microgravity that messes with blastocysts and their ability to implant and grow, but they do have ideas. Like, embryonic stem cells are part of the equation.

These are the cells in the blastocyst that differentiate, or diversify into all of the different kinds of tissues in the fetus. Microgravity appears to make them more resistant to differentiating into those more mature cells. It's possible that microgravity interferes with DNA methylation, a process known to affect cell differentiation.

Or there could be other factors — like how microgravity messes with the behavior of fluids, and how cells float in fluids. So, at this point, it's up in the air whether space pregnancy is feasible, outside of inventing prolonged artificial gravity. That said, even if we did figure that part out, there's evidence that a fetus couldn't develop in a healthy way in space.

After all, gravity isn't only important for the earliest stages of pregnancy. Scientists are pretty sure that it's also important in the third trimester, when it helps the fetus develop its muscles, including in the heart. Gravity may even play key roles in the development of synapses in the brain and sensory tissue in the inner ear.

And based on evidence from pregnant mice, it seems like microgravity during orbital spaceflight affects the development of vestibular functions — basically the sense of balance and motion. So, not great news. And even if we can compensate for microgravity, developing radiation shielding technology strong enough to protect a fetus is a whole other matter.

Outside of Earth's magnetic field, the effects of solar radiation are a lot stronger. And we've learned that the developing brain is extremely sensitive to radiation exposure, which can cause DNA damage, brain defects, and increased incidence of cancer. So, while there's a lot we still don't know about how this all works, and while we'll need many more studies in mammals, one thing's for sure: We may get there someday, but there are a lot of small steps to come before humanity can make the giant leap to pregnancy in space.

If you like deep dives into incredibly specific questions like this one, and absurdly competitive trivia, and also science poetry, then boy do we have a podcast for you. It's SciShow Tangents! Produced by Complexly and WNYC Studios, it's made by the same people who bring you.

SciShow! Well, some of them. Lately, they've been recording from home in the name of social distancing, which has involved host Ceri building a blanket fort.

Geez, who doesn't have a fully equipped podcast studio in their home already? Get with the program! Check it out at the link in the description -- and thanks for watching {♫Outro♫}.