Having a baby changes everything  …including your cancer risk.

And if that’s not bizarre enough for you at  face value, the way it all shakes out will be. See, cancer risk can go one of two ways.

You can become more likely to  develop cancer, or less likely. But in this case, it’s both. The same person can have an  increased and decreased risk of developing breast cancer after childbirth.

Confused? I’ll explain. Hopefully by the end, the baby will be  the only thing keeping you up at night. [♪ INTRO] A lot of things are in flux during pregnancy.

Hormones are changing your body,  from your joints to your blood flow. And it turns out that those  changes are related to a new risk of developing breast cancer later in life. Great… Now, some of the trends researchers have  observed in the relationship between childbirth and breast cancer risk are also  true for ovarian and endometrial cancers.

But most of the research is  focused on breast cancer, as one of the most common cancers in women. So this video is about your risk  of developing breast cancer. But know that what I’m saying may  also be relevant for other cancers.

Also, you don't have to be a woman to get pregnant, or have breast cancer, but the research says women, so we're going to say women. You get it. And since we’re talking about breast cancer, let’s just start by addressing  the melons in the room.

In pregnancy, your breasts may get bigger as early as the first trimester  to get ready to feed your baby. Throughout pregnancy, you ramp up  development of small branching ducts that will eventually produce milk. And for that to happen, a lot of  cells need to change and move around.

Some cells specialize to  make and secrete the milk, some make up the lining of the newly formed ducts, and others form new capillaries that  feed the fresh tissue with blood. And that’s probably all  happening thanks to the hormones that you might have noticed  when you accidentally came home from the market with almond milk instead  of oat milk and burst into tears. A peer-reviewed publication from  2019 found that the relationship between pregnancy and some of  the changes in breast cancer risk mainly comes from a specific type of  breast cancer, which also happens to be the most common type, called estrogen  receptor positive breast cancer.

So that’s one reason researchers think  hormones like estrogen are to blame. It’s also because of just how intertwined they are with the processes  that could promote tumor growth. See, your pregnant body makes more hormones like prolactin that encourage epithelial  cells to build out your ducts.

Estrogen and progesterone also trigger  those breast cells to multiply. Which is a good thing, because  you need to feed your baby. But some scientists think that this  increase in reproductive hormones also increases your risk of  cancer, because they stimulate the growth of cells that were already  poised to head down a cancerous path.

With so many growing, moving, and  replicating cells, any DNA that was damaged before or during that process can duplicate  and spread with every cell division. So what might have been a small  problem that never turns into anything starts showing up all over the  place and becomes a big red flag. At the end of the day, lots of  genetic damage can lead to tumors.

And that doesn’t just come from pregnancy,  but also extends to your postpartum body. Even when lactation ends, the remodeling  that takes place in breast tissue to stop making breast milk could likewise create  just the right environment to grow tumors. Researchers say your cancer risk reaches a  high point relatively soon after pregnancy.

They call it a short term risk. But when they say “short  term,” they’re talking about a peak risk around four or  five years after childbirth. If you’ve raised a kid, maybe  you feel like you blinked and all of the sudden your infant  is a full fledged kindergartener.

But an increased risk of developing  breast cancer for multiple years isn’t usually what I think of as  a short term health situation. Especially when you find out that it may  be a peak risk at 5 years, but your risk is still high and may be slowly returning  to baseline over a period of more than 20! You don’t even have a kid at home anymore and they’re still calling it a  short term effect of childbirth.

In any case, after pregnancy, your  chances of getting cancer go up. …But then they drop. And after a while, they end up lower than  someone your age who has never given birth. This SciShow video is supported by  Brilliant, the online learning platform with thousands of lessons in  computer science, math, and science.

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And you’ll get your first 30 days free! In pregnancy, you have a lot  of cells growing and changing, which contributes to your  unstable cancer-prone situation. But it’s also making things  way more stable in the long run by increasing the number of  new healthy breast cells.

These new cells used to be stem cells. And stem cells are more easily swayed  to become other kinds of cells. So while they’re very handy when  tissues need to grow and differentiate, they’re more vulnerable to the  kinds of damage that breed cancer.

Cells in a stable, differentiated  form are less susceptible to cancer. Hence the lower risk well  after a full-term pregnancy. So there are scientific  explanations for why your risk of developing cancer changes after childbirth.

But it’s not just based on  hormone and cell studies. There’s a real link between childbirth and cancer that comes from massive longitudinal  studies where researchers document the outcomes of tens of  thousands of people for decades. And they factor in things like family history to control for a generally  elevated breast cancer risk.

A collaboration of researchers  in Sweden, the US, and Italy combed through the cancer registry  and fertility registry in Sweden, looking at more than 12,000 cancer  patients and more than 62,000 people of the same age who had not  been diagnosed with cancer. They separated out the data based  on if and when everyone gave birth and found an increase in breast cancer  among people who had given birth for about 15 years after the event,  but then a decrease after that. And that effect is not short term.

In fact, the protective benefit of  a previous birth seems to compound when you give birth to more  babies after that first pregnancy. Researchers think that because  hormones are key players in this whole childbirth-cancer  relationship, by spending more time pregnant, you’re reducing your long-term cancer risk levels. Every period of full-term pregnancy and  longer lactation reduces the overall number of menstrual cycles, and your  lifetime exposure to hormones.

But there’s another twist. The decreased cancer risk tends to only exist if you have your first baby before around age 35. After that, you're entering years when  people tend to have more cancers in general so you can't take advantage of that decreased risk.

In that case, your risk is higher than  someone your age who never gave birth. To put it all together, pregnancy initially  increases your risk of breast cancer, then eventually lowers it, but only if your first pregnancy  was before the age of roughly 35. If you’re in your 30s watching  this video and worrying about your impending breast cancer diagnosis  because you haven’t given birth before, know that you’re probably not  growing breast cancer right now.

As a general rule, breast  cancer isn’t terribly common in women of reproductive  age, with or without babies. And there are things you might be able  to do, like breastfeeding for a year that helped to decrease breast cancer  risk by over 4% in a 2022 meta-study. More importantly, we’re talking about  risks and percentages, not a sure thing.

If having a kid is in the  cards for you, then great! And if not, also great. But, don’t get pregnant just  to try to avoid breast cancer.

Because you might only reduce your  risk of breast cancer by a little bit, but increase your chance  of having a baby by a lot. [♪ OUTRO]