40 Is Not "The New Fertile." Here's The Data Nobody Wants To Show You.

In a recent Glamour interview, Miller, now 44 and pregnant with her third child, reflected on her experiences. She had her first daughter at 29, her second at 42, and is now pregnant with this one at 44. “Having had a baby at 29, and then having a baby at 42, and now 44, it's so much easier… We don't judge men who are having kids in their 80s. Why on earth is there any sort of narrative?” she said. She called being pregnant in her forties “the best,” describing a grounded peace that her thirties lacked.

It’s a lovely sentiment, if you already have the baby. But Miller is adding fuel to a narrative that has been spoon-fed to millennial and Gen Z women for decades: put off love, put off family, become someone first, and then have a baby in your forties. It’ll be easy. You’ll be more “you.” Society doesn’t judge men who become fathers in their eighties, so why the fuss?

Well, Sienna… women can’t have babies in their eighties, for starters. There’s a thing called biology.

Scrolling Instagram the same week, I was served a post declaring “40 is the new fertile.” Fitness influencers and wellness accounts are flooding feeds with the message that age is just a number, your body can do anything if you optimize hard enough, and late motherhood is empowering. The comments are full of hopeful hearts and “this is my sign” captions. It’s dangerous misinformation dressed up as girlboss affirmation.

Fitness influencers and wellness accounts are flooding feeds with the message that age is just a number, your body can do anything if you optimize hard enough, and late motherhood is empowering.

I know because I lived it.

I waited too long. Like so many women of my generation, I bought the story that my twenties and early thirties were for climbing ladders, traveling, finding myself, and not “settling” with babies of my own. I dated casually, then I was engaged to a man who already had children and didn’t want more. I built a career. I told myself fertility was something I could deal with later if I changed my mind—maybe freeze eggs at 35, maybe try at 38. The door, I assumed, would stay open until I was ready to walk through it.

It didn’t.

By the time I seriously tried to go down the family path in my mid-thirties, life had other plans. Months turned into years of singlehood and I experienced the grief that comes when biology says “no” louder than any life coach ever said “yes.” I’m now in my late thirties, and that door has closed. Not slammed. It closed gently, with the finality of aging health and the harsh statistics no one wanted to say out loud.

Here’s what the data actually shows.

Female fertility peaks in the early twenties and begins a gradual but significant decline around age 32. It drops more sharply after 37. According to the American College of Obstetricians and Gynecologists (ACOG), healthy couples in their twenties and early thirties have about a 25 percent chance of conceiving in any given menstrual cycle. By age 40, that drops to roughly 10 percent.

Miscarriage rates tell an even starker story: around 10-11 percent for women in their twenties and early thirties, rising to 17 percent by 35-39, 33 percent by 40-44, and over 50 percent after 45. What Sienna isn’t telling women is that they have a 1 in 2 chance they’ll have a miscarriage if they try for a baby at her age.

What Sienna isn’t telling women is that they have a 1 in 2 chance they’ll have a miscarriage if they go for a baby at her age.

Pregnancy itself becomes riskier. Women over 40 face significantly higher odds of gestational diabetes, preeclampsia, placenta previa, placental abruption, preterm birth, low birth weight, cesarean delivery, and postpartum hemorrhage. Maternal complications and, even the small but real risk of stillbirth, climb with every year past 35.

And then there’s assisted reproduction. Many women hear “IVF” and assume it’s a reliable backup plan. It’s not—at least not with your own eggs after 40. CDC data on live birth rates per IVF cycle show success hovering around 55 percent for women under 35, dropping to about 40 percent for 35-37, 26 percent for 38-40, and falling sharply from there. After 42 or 43, the numbers become single digits for most clinics when using a woman’s own eggs.

Donor eggs can dramatically improve outcomes, but that’s a different emotional and financial path, not the simple “try later” story being sold.

These aren’t scare tactics. They’re biology. Ovaries don’t read Glamour or Instagram captions. Egg quantity and quality decline relentlessly, and no amount of kale smoothies or mindset shifts changes the fundamental timeline.

Sienna Miller is not lying about her own experience. She feels more grounded now. She isn’t juggling the same career pressures she faced at 29. But she’s also extraordinarily privileged. She is wealthy. She has access to the best medical care money can buy—private specialists, cutting-edge monitoring, and, if needed, treatments most families can’t afford. (A single round of IVF in the U.S. routinely costs $15,000–$25,000 before medications.)

Egg quantity and quality decline relentlessly, and no amount of kale smoothies or mindset shifts changes the fundamental timeline.

She can hire night nurses, nannies, and housekeepers so that chasing toddlers doesn’t exhaust her the way it would an older woman working two jobs or relying on daycare waitlists. Most of us don’t have that safety net. Celebrities do.

Their voices carry because they’re loud and glamorous, but their realities are not representative. When they normalize late motherhood without acknowledging the medical odds, the financial barriers, or the silent heartbreak of the women who try and fail, they do real damage. Young women hear “it’s easier” and assume the biological part is easy too. It isn’t.

I’ve written before for Evie and for the Institute for Family Studies about my experience. The cultural script that told us to prioritize everything else first sounded liberating. Yet it left many of us grieving the children we never got to meet. The data backs this up: fertility rates for women under 30 have fallen while rates for women in their late thirties and early forties have risen—not because biology changed, but because cultural messaging did. But that doesn’t mean it’s healthy, easy, or even, dare I say, responsible.

We need to tell women the truth, even when it’s uncomfortable. Yes, some women conceive naturally at 42 or 44. Miracles happen. But they are exceptions, not the new normal. Planning your life around the exceptions is how you end up where I did: accepting the results of your choices, yet wondering what would have happened if you started earlier when the odds were still in your favor.

When they normalize late motherhood without acknowledging the medical odds, the financial barriers, or the silent heartbreak of the women who try and fail, they do real damage.

This doesn’t mean women in their thirties or forties should despair. It means they deserve honest information so they can make informed choices, and we should educate and warn the next generation of young women. Freeze eggs earlier if you must delay. Be realistic about timelines. Prioritize finding a partner who wants a family sooner rather than later. And stop pretending that men’s ability to father children into old age somehow erases women’s much narrower window. Sperm quality declines too, but men keep producing it. Women are born with all the eggs they’ll ever have, and those eggs age with us and sometimes tap out.

The “40 is the new fertile” crowd wants to make biology sound optional. It isn’t. Love, marriage, and motherhood are not side quests to be slotted in after the career chapter. They are core to many women’s deepest longings, and delaying them comes with costs the other glossy magazines rarely mention.

Sienna Miller can enjoy her third pregnancy in style and peace. I genuinely wish her well. But the rest of us, especially the next generation scrolling Instagram, deserve better than fairy tales. We deserve the truth: your forties can be wonderful, but they are not the new twenties when it comes to making babies. The data is clear. The window is real. And pretending otherwise leaves far too many women mourning what might have been.