One day, a woman is spending her Saturdays doing her normal Saturday stuff—blueberry pancake brunch, curling up on the couch with the cat reading a novel, grabbing a beer with friends. By the next, her life is suddenly and completely about keeping a screaming, floppy, red-faced, cone-headed thing alive using fluids secreted from her chest. Happy birthday to that.

Scientists and philosophers have spent quite a bit of time thinking about what that night-and-day transition of childbirth means for mothers, who (often along with a partner) go from having one kind of life to another in less than a year. Philosophers have labeled childbirth a “transformative experience,” arguing that because a parent literally cannot know what it will be like to have a child until it happens, they can’t make informed decisions about whether to have a baby or not. Some other transformative experiences by philosophers’ estimation: gaining the ability to hear, emigrating to a new country, going to war.

What all four of those experiences have in common is that they can alter the brain—dramatically, and maybe irreversibly. After pregnancy, the many traces of having become a mother are littered throughout the body, and biologists are just starting to tease apart what they are and what they might mean.

Shrunken Brain

We’ve known for a long time that motherhood changes you—duh, right?—but scientists are just starting to characterize the precise nature of those bodily and psychological changes. Scientists have known for a couple of decades that women’s brains shrink during pregnancy. I know what you’re thinking: Oh, great. One recent study started to get a handle on how long those grey matter reductions stick around. The shrunken brain areas were still small two years after childbirth, suggesting that the changes are there for good. It’s just the first study of its kind, but Liisa Galea, a neuroendocrinologist at University of British Columbia who was not involved in that study, isn’t surprised. “It shouldn’t be so shocking that pregnancy, and the experience of having and caring for a baby and then child, is going to permanently alter your brain.”

The researchers also looked at where precisely those shrinkages happen. It’s a bunch of areas in the front of the brain that are thought to help people understand the minds of others—the “theory of mind” bits. “That reduction in brain size sounds bad, but was actually related to better mother-child interactions,” says Galea. The running hypothesis is that shrinking might actually be a sign of the brain honing its circuits for mothering. The researchers didn’t see shrinking in dads.

Hormonal On Switch

A shrinking brain isn’t the only counterintuitive effect of pregnancy and childbirth. While a pregnant woman might get “baby brain”—getting in the car and forgetting where she’s going, or forgetting her boss’s name—the reality of how a woman’s memory and thinking abilities change during and after childbirth is a lot more nuanced. Some studies do find that memory and spatial navigation are impaired in pregnant women, but others find that they’re not. One duo of researchers suggested that these experimental discrepancies come from the fact that only women pregnant with girls have cognitive impairments, perhaps because their levels of the hormone human chorionic gonadotropin are higher. Cognition seems to bounce back to normal levels a year or so after the baby is born (possibly correlated with getting some damn sleep).

Of course, motherhood doesn’t end after a baby turns one. “It’s not like the changes stop once the children are older,” Galea says. She studies rodent moms, and has found that they’re better at mazes than non-moms well after the pups start to become independent—a sign of enhanced cognitive abilities.

These changes in brain size and functioning probably come from the crazy levels of hormones coursing through a pregnant woman’s system. Throughout pregnancy, steroid hormones like progesterone climb higher and higher. “Depending on the hormone, you’re talking about 200 to 300 times normal levels for months at a time,” Galea says. Lots of brain areas have specialized receptors for steroid hormones, including the areas involved in memory and emotions. Those hormones may act as a big old ON switch for maternal behavior, setting a mom’s brain up for the task of keep the kid alive. “You need to learn a host of new behaviors in order to ensure the survival of your offspring,” Galea says.

Hormones jump-start the process, but they can also overshoot—sometimes causing mental illness. Researchers aren’t sure yet why postpartum depression and obsessive compulsive disorder risks spike after pregnancy, though it might be caused by the crazy drop in hormones after childbirth or the stress of round-the-clock baby care. There’s also the hypothesis that hyper-vigilance and obsessive behaviors that could be useful to new mothers in some contexts have gone awry.

Those hormone changes in pregnancy leave traces after the baby is born. Once a woman has done the whole act-of-creation-in-the-uterus thing once, her hormone levels throughout her menstrual cycle are lower than in pre-pregnancy cycles. They’re a lingering tag on the body that might help to lower rates of some types of cancer as moms age—or serve as a constant signal to keep the brain in mom-mode.

Fetal Invasion and the Long Long Term

Those aspects of how pregnancy changes the brain might not be too surprising, but this where things get weird. During pregnancy, the developing baby sloughs off cells, which take a ride through the placenta to a mother’s bloodstream. And then those cells are there, hanging out in the brain, bones, pancreas, wherever—some of them until mom dies. In rodents, scientists dyed some of those fetal-derived cells and saw that they had turned into fully-fledged neurons and integrated into parts of mom’s brain. For rodent mothers who’ve had boys, that means there were neurons with Y chromosomes in there.

This is a new area of research, so scientists aren’t totally sure yet what the presence of these sneaker cells—called fetal microchimerism—means. One option is that they don’t really do anything interesting. But the other option, still a hypothesis for the most part, is pretty neat. A study that looked at the brains of people after they had passed away found that moms with more of their children’s cells in their brains were less likely to have anatomical signs of Alzheimer’s disease. The hypothesis from rodent studies is that the fetal cells might jump in and replace mom’s tissue, acting as a source of stem cells to replace mom’s old or diseased cells. Then again, other studies suggest that those cells just muck things up, causing immune responses. It’s a budding field.

Either way, next time you think about the horrible day that your mother will die, remember: Some of you will be dying too, the part of you that is still parasitizing your mom’s body all these years later. Yes, the moral of this article is to appreciate your mother.