This episode of SciShow is supported by Brilliant.

To learn more about their Mathmatics Fundamentals course, you can go to Brilliant.org/SciShow. [ ♪INTRO ]. Human pregnancy typically lasts nine months.

From when a sperm fertilizes an egg, to when a baby is born is roughly 38 weeks later. But some mammals do things differently: They can pause their pregnancies to put an embryo's development on hold. It's called embryonic diapause.

And if you're wondering how that works… so were scientists until recently. Pausing pregnancy after conception might seem weird, but it happens in more than 130 mammal species, from mice to seals. It occurs early, during the blastocyst stage, when the embryo is a quickly-dividing ball of cells.

Normally during this stage, the embryo embeds into the uterine wall, where it keeps growing and becomes a fetus. But in diapause, this gets delayed. The embryo just hangs out in the uterus, developing slowly or not at all.

And it can stay that way for months, or even a year. Eventually, though, the egg does implant, at no harm to the pregnancy. This is a really useful trick, and it happens for one of two reasons: either to help an animal produce the most offspring, or to delay birth until conditions are right for the newborn to thrive.

That first mechanism is pretty simple: When an animal is nursing, they experience a rise in hormones that prevents embryos from implanting. This is common in mice and some marsupials, and it gives older siblings time to wean off their mother. That second mechanism, though—that one confused us for a while.

We knew it was common. It happens all the time—mostly in carnivores, since they tend to endure harsh climates and food shortages… times that are not great to have a baby. In some cases, it even happens with every gestation.

We also knew it isn't a conscious decision. Instead, it's controlled by environmental cues, like hunger or temperature. But only recently did we begin to learn how it really works.

We've started to understand that this kind of diapause happens because those cues affect the amount of mTOR. mTOR is an enzyme that's sensitive to a variety of molecules, like oxygen, glucose, and amino acids—things needed for cell growth and multiplication. It helps organisms regulate their growth based on how many nutrients are available. And since it's sensitive to so many molecules, it can detect when a cell's nutrient levels are low.

Normally, this is important for growth in general. But around 2016, we began to find that this enzyme also plays a big role in diapause. Like, in a newer experiment published in 2020, researchers found that starving mouse embryo cells led to changes in the cells that suppressed mTOR.

And when that happened, the embryos went into a state that looked a lot like diapause. So most likely, this enzyme is also affecting this process in nature. When something like a food shortage hits, the lack of nutrients might cause a drop in mTOR, and boom—you got a paused pregnancy.

Now, we still have a lot to learn about exactly how this works—because humans also have mTOR, and we cannot pause our pregnancies like this. So there has to be something else going on. But this trick is helpful and convenient for the animals that can do it.

And maybe someday, we'll find out if this biology can also help us. If you want to keep learning this summer, you might enjoy one of Brilliant's courses—like their newly-designed Mathematical Fundamentals course. It will not teach you how to do embryonic diapause, but it does have new illustrations, interactive features, and challenges.

And like all of Brilliant's courses, it's available offline with their iOS and Android apps. So even if you have a rough internet connection, you can still check it out. To learn more about the course, you can go Brilliant.org/SciShow.

And the first 200 people to sign up using that link will get 20% off an annual Premium subscription. [ ♪OUTRO ].