Is the sex of your child really just a coin toss? For generations, we’ve accepted the idea that every pregnancy comes with an even 50-50 shot of producing a boy or a girl. But new research suggests that might not be the full story.

A study published in Science Advances challenges this long-held belief and adds a fascinating new layer to our understanding of human reproduction. Drawing from data spanning nearly 60 years and over 58,000 pregnancies, researchers found compelling evidence that the biological sex of children may sometimes run in families — and that age, genetics, and possibly other overlooked factors could nudge the odds away from that supposedly fair coin flip.

The 50-50 Myth?

At first glance, the logic behind the 50-50 assumption seems solid. Sex in humans is determined primarily by whether the sperm that fertilizes the egg carries an X chromosome (resulting in a girl) or a Y chromosome (resulting in a boy). Since roughly half of a man’s sperm carry each chromosome, it stands to reason that the odds of having a boy or a girl should be equal — right? Not exactly.

Dr. Jorge Chavarro, a reproductive epidemiologist at Harvard T.H. Chan School of Public Health and senior author of the study, wasn’t so sure. Along with PhD student Siwen Wang and a team of collaborators, Chavarro analyzed pregnancy and birth data from the long-running Nurses' Health Study — a landmark series of investigations into women’s health dating back to 1976. They focused on a group of more than 58,000 women who had at least two children.

The team noticed something striking: far more families than expected had multiple children of the same sex — all girls or all boys — especially among women who had their first child later in life. These clusters couldn’t be explained by chance alone.

In other words, some families seemed “tipped” toward having one sex over the other — and the tilt wasn’t always subtle.

Why Age Matters But Not In the Way You Think?

One of the clearest signals in the study was maternal age. Women who had their first child after the age of 28 were about 10% more likely to have all girls or all boys than women who started their families before age 23. It’s a modest but statistically significant increase that held up even after adjusting for various other factors.

It comes down to the biology of reproduction and how it changes over time. As women age, they undergo shifts in the vaginal environment and reproductive hormones that could influence which sperm — X or Y — are more likely to reach and fertilize the egg. For instance:

The vaginal environment may become more acidic with age, and X-carrying sperm (which are slightly larger and more chemically robust) may survive better in such conditions.

The first phase of the menstrual cycle, called the follicular phase, tends to shorten with age. Some researchers believe this could create changes in cervical mucus and oviduct fluid that favor Y sperm instead.

These competing factors mean the impact of aging on sex outcome may not be uniform — it could vary depending on individual biology. But the net result is that older maternal age seems to increase the odds of having children all of the same sex.

Another eye-opening part of the study was its genetic analysis. Researchers examined genetic data from a subset of over 7,500 women and found two specific gene variants that were significantly associated with single-sex offspring.

One gene variant (located near NSUN6) was linked to having all daughters.

Another (near TSHZ1) was associated with having all sons.

These genes aren’t currently known to affect fertility or reproduction directly, and their exact roles remain unclear. But their presence suggests a biological basis for sex-skewed births in some families — an area ripe for future research.

In short, while we’ve long believed sex determination is a random roll of the dice, some people might have a “weighted coin” — without ever realizing it.

Could Behavior Be Driving the Pattern?

Some might wonder: what if this pattern is just driven by behavior? For instance, parents who have two boys might keep trying until they have a girl, leading to more same-sex siblings by default. The researchers considered this too.

To rule out this explanation, they ran an analysis excluding the last birth in each family — the one most likely to be affected by the decision to stop having more children after getting “one of each.” Even then, the same-sex clustering held strong.

That suggests something deeper is going on, rooted in biology rather than just human choice.

What This Means for Expecting Parents?

So, what should couples take away from this? First, there’s no need to overthink your chances if you’re planning to start a family. Across the entire population, the average likelihood of having a boy or a girl still hovers close to 50%. But for some individual families, those odds may be subtly skewed by age, genetics, and biological quirks we’re only beginning to understand.

Importantly, the study also highlights just how much we still don’t know about sex determination and human reproduction. It opens new avenues for exploring how maternal and paternal factors interact — and how genetic and environmental forces shape outcomes in subtle but meaningful ways.

Wang and her team hope to replicate the findings in more diverse populations and include paternal data in future analyses. Since most participants in this study were white and from the U.S., it’s not yet clear how these patterns hold across other racial, ethnic, and geographic groups.

This research doesn’t rewrite the biology of reproduction — but it does suggest that some old assumptions might be oversimplified. Sex may still be determined by the X or Y chromosome in sperm, but the journey to conception is influenced by a dynamic, personal landscape of biological factors.

What the study captures is a new lens: a shift away from the purely statistical view of childbirth toward a more personalized understanding of how life begins.

And perhaps, for families that have always wondered why they have “just boys” or “just girls,” the answer might be: it’s not just chance. It might be part of your biological story written long before the baby arrives.

Inner Child’ is Health and Me's new mental health series where we deep dive into lesser-known aspects of child psychology and how it shapes you as you grow up. Often unheard, mistaken, and misunderstood, in this series we talk about the children’s perspective and their mental health, something different than you might have read in your parenting books. After all, parenting is not just about teaching but also unlearning.

A new study from Durham University has found that children as young as five years old begin to form their perception of what a "normal" body looks like based purely on visual exposure, and that this perception can be subtly, but powerfully, reshaped by what they repeatedly see around them.

Led by Professor Lynda Boothroyd from Durham's Department of Psychology, the study reveals that just like adults, children’s brains adapt quickly to the images they are exposed to.

In the experiment, children aged 7 to 15 were shown a series of body images and asked to rate their heaviness. After viewing either thin or heavy body types repeatedly, their perception of what was “normal” shifted.

Those who saw heavier figures began rating the same bodies as lighter than they had before — indicating that their internal “model” of a normal body had recalibrated.

This shift wasn’t limited to older children. The youngest participants, seven-year-olds, too, showed the same level of perceptual flexibility as adults.

The implications are significant: it suggests that by early primary school, and likely even before, children’s views about bodies are already being shaped by their environment, including toys, cartoons, advertisements, and even family comments.

The Seeds of Self-Image: What Shapes Children Early

Dr. Sameer Malhotra, Director and Head of the Department of Mental Health and Behavioral Sciences at Max Super Speciality Hospital, Saket, explains that children as young as three to five years old begin internalizing messages about body image, and often in ways adults may not even notice.

“Parental language plays a huge role,” he says. “When a parent casually says, ‘I feel fat today,’ or makes dieting comments, children absorb that. Even if the comment isn’t directed at them, it registers.” Children also learn by imitation. This means they pick up patterns of behavior, tone, and attitudes from parents, siblings, peers, and media.

Toys and cartoons often carry these subtle messages too.

Dolls with tiny waists, heroes with exaggerated muscles, or villains portrayed as overweight, all reinforce stereotypes.

“Children's media tends to link thinness with success, beauty, or goodness,” Malhotra adds. “These associations are laid down before a child even begins formal schooling.”

Words That Stick: The Power of Casual Comments

Even offhand remarks like “Don’t eat too much, you’ll get fat” or calling someone “too skinny” can significantly influence a child's self-image. Dr. Malhotra points out that such language can link appearance to self-worth, foster guilt around food, and create anxiety about acceptance.

“Children mirror parental opinions. If they hear fat-shaming or appearance-based criticism, they begin to judge themselves and others the same way,” he explains. And these judgments don't remain in childhood, they form the blueprint for how a person thinks about their body throughout life.

What Can Be Done?

While this conditioning starts young, it can also be gently undone. The Durham study highlights the power of simply seeing more diverse body types. According to Dr. Malhotra, both homes and schools can play a big role in broadening what “normal” looks like.

“Including books, posters, and videos that feature people of different body types — not just the slim or athletic ones — helps kids understand that bodies come in all shapes and sizes,” he says.

He recommends:

Modeling balanced behavior at home: Focus on eating healthy “to feel strong” rather than “to stay slim.”

Media literacy: Teach children to question what they see in TV, ads, and social media. Help them understand that not everything shown is real or ideal.

Affirming non-appearance traits: Compliment children on their creativity, kindness, strength, or curiosity, not just their looks.

Encouraging open conversations: Avoiding the topic of body image can make it more taboo. Instead, create space for kids to talk about their feelings and the pressures they may face.

Parents: The First Role Models

One of the most effective strategies, according to Dr. Malhotra, is for parents to be mindful of their own body image talk.

“If you stand in front of the mirror criticizing your appearance, your child learns that bodies are to be judged,” he explains. “But if you focus on how your body helps you run, cook, or hug your child, that teaches gratitude and respect.”

The Durham study may have focused on how children see, but it’s what they hear and feel, especially from the adults around them, that gives those images meaning.

In a world of filtered faces and digitally enhanced bodies, helping a child grow up with a grounded, diverse sense of beauty may start with something as simple as changing the channel — or the conversation.

In a startling attempt to reverse its plummeting birth rate, Russia has expanded its state maternity incentive to include pregnant teenage schoolgirls, offering them over 100,000 roubles (around ₹1.1 lakh) to carry pregnancies to term and raise their children. The controversial policy, rolled out in 10 regions in recent months, follows a nationwide demographic strategy initiated in March 2025 that originally applied only to adult women.

The move is aimed at addressing Russia’s birth rate crisis: in 2023, the average number of births per woman was 1.41, well below the 2.05 needed to maintain population levels.

President Vladimir Putin has often tied Russia’s greatness to its size, in terms of both land and people, and views population growth as a marker of national strength. Ironically, the war in Ukraine, which has drained Russia’s youth through deaths and displacement, has also contributed to the demographic downturn.

However, by placing the responsibility of national recovery on teenage girls, health experts warn, the country may be plunging an already vulnerable group into serious and avoidable harm.

What the World Health Organization Says

According to the World Health Organization (WHO), adolescent pregnancies, particularly in girls aged 10–19, carry significantly higher health risks compared to adult pregnancies. These include life-threatening conditions such as eclampsia, puerperal infections, and systemic complications during childbirth. The children of teen mothers also face higher odds of low birth weight, premature birth, and neonatal complications.

Globally, adolescent pregnancy accounts for a troubling portion of maternal deaths. In fact, the leading causes of death among girls aged 15–19 worldwide are complications from pregnancy and childbirth.

And yet, rather than protecting schoolgirls from these outcomes, Russia’s current policy may be incentivizing them.

Teen Bodies, Adult Consequences

Medical experts, including Dr. Ekta Singh, who works at Lybrate, an Indian mobile health communication and delivery service, who is a gynaecologist with 29 years of experience, caution that teenage pregnancies can be catastrophic for both the mother and child. “Girls under 15 face higher maternal mortality,” Dr. Singh writes, noting the elevated risks of anemia, hypertension, STDs, and organ malformations in the fetus.

Additionally, mental and social consequences are deeply interwoven: many teenage mothers are forced to drop out of school, remain single, and are pushed into poverty, with little access to adequate healthcare or family support systems.

According to a 2022 review published in Cureus, unwanted teenage pregnancies are closely linked with stillbirths, spontaneous abortions, preterm labor, and obstructed deliveries, sometimes resulting in maternal death due to the immaturity of the pelvic structure.

Teen mothers are also less likely to access timely prenatal care, either due to shame, stigma, or sheer lack of awareness. Many don't even realize they're pregnant until the third trimester, by then, critical stages of fetal development have already passed without necessary medical supervision.

A Nation's Future on Girls’ Shoulders

While 43% of Russians reportedly support the new policy, and 40% oppose it, cites a recent survey by the Russian Public Opinion Research Centre. What remains alarming is the human cost of this demographic experiment. The payment, which may seem like a windfall to families struggling with poverty, could, in reality, be bait for systemic harm.

The WHO emphasizes that preventing adolescent pregnancy is essential to improving maternal and newborn health worldwide. That means better sex education, accessible contraception, and societal support to delay early pregnancies, not rewarding them.

Meanwhile, the 2022 Cureus study underlines how teenage pregnancies contribute to a cycle of intergenerational poverty, poor health outcomes, and educational dropouts. Babies born to teen mothers are more likely to become teen parents themselves, continuing the same pattern of vulnerability.

Paying teenage girls to have babies may temporarily raise Russia’s birth figures, but it ignores a painful truth: adolescence is not a time for forced motherhood.

When Roshan George and Julie Kang were told during IVF testing that they both carried a rare gene linked to early-onset hearing loss in infants, they didn’t hesitate to dig deeper. Like many in San Francisco’s tech scene, they turned to data. It was not just any data, but a full genomic analysis of their embryos from a start-up called Orchid.

It cost them $30,000 to screen 12 embryos. Six were viable. They sorted through risk scores for everything from bipolar disorder to type 2 diabetes, eventually selecting the embryo with the best odds. Their daughter, Astra, was born this March. She was healthy and was born with perfect hearing.

What this couple did might sound like a glimpse into the distant future. But in Silicon Valley and beyond, it’s already happening.

Also Read: 'Three-Parent-Baby' Technique: How UK Is Making Healthy Babies With DNA From Three People To Avoid Genetic Mitochondrial Disease

Silicon Valley’s Fertility Revolution

Orchid, a fertility start-up based in San Francisco, reports the Washington Post, is spearheading a radical new chapter in human reproduction. Founded by Noor Siddiqui, the company offers whole-genome sequencing of embryos created through IVF, allowing parents to screen for over 1,200 single-gene disorders, and to assess the embryo’s genetic risk for more complex diseases like cancer, schizophrenia, and Alzheimer’s.

In Siddiqui’s words, the goal is to help build “a generation that gets to be genetically blessed and avoid disease.”

At $2,500 per embryo, in addition to the roughly $20,000 average cost of one IVF cycle, the service is far from accessible to everyone. But in data-driven tech hubs like San Francisco and Austin, it’s catching on fast.

Sex Is For Fun. Embryo Screening Is For Babies

“Sex is for Fun. Embryo Screening is for Babies.”

That’s how Siddiqui sums up the shifting mindset. She imagines a future where sex is decoupled from reproduction, and where couples routinely pick their children using spreadsheets and algorithms. Sounds too robotic? But it really isn't even a distant future, it is slowly becoming reality.

At a private dinner party in Austin last spring, Siddiqui pitched her vision to a group of women sipping mocktails and wearing pastel baseball caps with one word: BABIES.

One of the attendees, reports the Washington Post, Shivon Zilis, who is a tech executive and mother to several of Elon Musk’s children, is reportedly an Orchid client. Sources say at least one of her children with Musk is an Orchid baby.

Siddiqui, now 30, plans to have four children herself using embryos screened by her own company.

The Promise of Prediction

So how does it work? Orchid uses five cells from an early-stage embryo to sequence the full human genome, a feat many geneticists previously believed impossible. It then uses machine-learning models to produce polygenic risk scores, predicting an embryo’s likelihood of developing certain diseases.

Backed by high-profile investors like Coinbase CEO Brian Armstrong and Ethereum co-founder Vitalik Buterin, Orchid is now in over 100 IVF clinics in the U.S.

The tech is built for a specific demographic: affluent, tech-savvy, data-worshipping individuals willing to invest in what one investor called a “genetic trust fund” for their future kids.

A Trust Fund of Traits Or Modern Eugenics?

The term eugenics makes many recoil. Siddiqui distances herself and her company from the idea, and insists the goal is about health, not perfection. But critics are wary.

When you screen for risk factors like schizophrenia, are you also unintentionally screening out traits tied to creativity? Could polygenic scores become a gateway to selecting for intelligence, height, or athleticism? And is this a privilege only the rich can afford?

Some scientists argue and the Washington Post reports, that these tools give an illusion of control. “It’s easy to moralize from an ivory tower when your child isn’t the one who might be born with a fatal disease,” said Orchid spokeswoman Tara Harandi-Zadeh.

Still, others worry about the science itself.

The Fault in the Genes?

One of Orchid’s biggest scientific claims is that it can sequence an entire genome from just five embryonic cells. This is hotly contested. Svetlana Yatsenko, a clinical genetics expert at Stanford, called it “basically Russian roulette,” citing errors introduced in the DNA amplification process.

Others are skeptical of the reliability of polygenic risk scoring, especially for traits like intelligence where predictive accuracy is extremely low, just a few IQ points at best. These risk scores also struggle with accuracy in non-European populations because the databases they draw from are largely Eurocentric.

Siddiqui acknowledges the challenge. Her husband is Middle Eastern, and she herself is South Asian. She says Orchid adjusts for population bias and, in some cases, doesn't offer scores at all.

No Rules, All Risks?

What’s striking is the lack of regulation. In the U.S., there are no real restrictions on what kinds of predictions companies like Orchid can sell. Their algorithms are proprietary and not vetted externally.

Despite the American College of Medical Genetics and Genomics advising against embryo screening for polygenic risks and calling the benefits “unproven”, companies are surging ahead. A newer Thiel-funded start-up, Nucleus, claims it can screen embryos for more than 900 traits and conditions.

As Peter Kraft, a Harvard expert on polygenic scores, put it: “The science doesn’t add up.”

Still, Parents Want Information

Despite the skepticism, demand is rising, notes the Washington Post. Many parents, especially those facing infertility or rare genetic conditions, are desperate for clarity. The George-Kang family didn’t expect certainty. They just wanted more information, more peace of mind.

“I think everyone who wants to have a baby should be able to have one,” Siddiqui told The Washington Post. “And they should be able to have a healthy baby.”

Critics might call it premature, or even dystopian. But to Siddiqui, it’s personal. Her mother lost her sight to a rare genetic disease. To her, it’s not just about tech, it’s about giving families a fighting chance.

The Future of Fertility

Whether Orchid’s vision becomes mainstream or remains an elite niche remains to be seen. But the door has opened to a future where reproduction is shaped not by nature, but by code, choice, and capital.

The baby of tomorrow may not be conceived in passion, but in planning and through lab reports, data sets, and algorithmic scores.

And in Silicon Valley, that future is already being born.