October is Breast Cancer Awareness Month, ladies. Time for pink ribbons, fundraising, mammograms, and that familiar list of lifestyle changes you can make to decrease your risk.

I bet most of you can recite it in your sleep: examine yourself monthly, watch your weight, exercise, eat berries, vegetables and fiber. Don’t smoke or drink excessive alcohol. Avoid red meat and fatty foods.

Those are the guidelines etched in our brains, women young and old, desperate to dodge the dreaded bullet that will strike one in eight of us. But as you stock up on blueberries and sauerkraut, please know one more thing. The “lifestyle” choice that provides the best protection from this epidemic has nothing to do with diet, cigarettes, or booze.

You won’t find it highlighted in women’s magazines or health websites, but it’s the mommy track that provides the greatest protection against breast cancer: giving birth before thirty, having a bunch of kids, and breastfeeding them—for a long time. Numerous large studies have shown that each birth will reduce your risk by ten percent and each year of nursing by at least four percent. So if you start your family early, have three kids and nurse them each for two years, you’ve decreased your risk by about 54 percent.

That’s dramatic, and much more protection than you’ll get from staying a size six and snacking on Brussels sprouts.

Here’s how it works: breast tissue is not fully mature on a cellular level, until the last trimester of pregnancy. Immature breast tissue is more susceptible to malignant change. When motherhood is delayed, the breasts remain immature and vulnerable for a longer period, giving more opportunity for cancer to develop. The mechanisms by which nursing confers protection are less clear, but are related in part to interrupting ovulation.

Of course, not every woman has the option or the opportunity to become mothers. Many prefer to spend their twenties and thirties in pursuit of other vocations, and others happily choose childlessness. But that’s beside the point. Not everyone is able to, or interested in, working out three times a week, or substituting salmon for their cheeseburger, but we instruct them about the benefits of such nonetheless.

October is Breast Cancer Awareness Month, right? It’s all about publicizing the facts, so women can make informed decisions. If we’re encouraging women to rethink their “lifestyles,” why omit the most critical lifestyle choice of all?

It’s another instance of ideology trumping science. Emphasizing the benefits of early motherhood could—gasp!—encourage some young women to give marriage more priority, and postpone their demanding career. They might decide it’s a diamond they most want now, not a PhD. Continued...