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 your 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 it 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've blinked and all of a sudden your infant is a full-fledged kindergartner.

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 five years, but your risk is still high and maybe 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.

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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 please 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.