Female Embryos at a Higher Risk of Death During Pregnancy, Study Suggests

Here’s a surprising truth: from the moment of conception, it’s not just chance that decides whether a pregnancy results in a boy or a girl—and the odds aren’t as fair as you’ve been told.

The common wisdom goes like this: when a woman gets pregnant, she has a 50-50 chance of having a boy or a girl.

It’s one of those “everyone knows” facts, so ingrained that we rarely stop to question it.

But if you zoom out and look at birth data worldwide, something curious appears.

For every 100 girls born, there are 103 to 107 boys.

That’s not just a fluke. It’s a global trend, seen across countries, cultures, and continents. So what’s going on?

The traditional answer has been rooted in the “fragile male” hypothesis—the idea that male fetuses and babies are inherently more vulnerable to miscarriage, disease, and accidental death, both inside and outside the womb.

To balance this out, nature supposedly produces more male embryos from the start, compensating for the higher male mortality rate.

But here’s where it gets interesting.

A massive new study has flipped this theory on its head.

Contrary to what we’ve believed for decades, male and female embryos are conceived in equal numbers.

And it’s not the boys who are more likely to die in utero—it’s the girls.

What the Data Really Says About Conception

Let’s begin where life begins—conception.

According to groundbreaking research led by biologist Steven Orzack of the Fresh Pond Research Institute, the human conception ratio is a true even split: 50% male, 50% female.

There’s no natural overproduction of males to start with.

Orzack and his team didn’t arrive at this conclusion lightly.

This wasn’t some back-of-the-envelope calculation or a narrow lab study.

It was a deep dive into human development, spanning multiple stages of pregnancy and using data from over 30 million pregnancies.

“It’s the largest compilation of data for this kind of investigation that’s ever been put together,” Orzack told NPR.

Their dataset included:

140,000 pregnancies tracked from week one, all via assisted reproductive technologies like IVF (allowing for precise conception timing).
900,000 fetal screening tests (such as amniocentesis).
30 million abortion, miscarriage, and birth records from across the US and Canada.

This massive trove allowed them to examine what really happens to embryos from the moment of fertilization to birth.

And what they found defied long-held assumptions.

What If It’s the Girls Who Are More Fragile?

For decades, biology textbooks have taught that male embryos are more likely to die before birth—too fragile, too genetically unstable, too prone to developmental issues.

This belief underpinned the logic that nature compensates by producing more males to begin with.

But what if that logic is completely wrong?

“This is an excellent example of an idea in science that’s had wide circulation, but where the empirical basis was ambiguous,” Orzack told The Guardian.

The new data suggests that male and female embryos begin life on equal footing.

But as pregnancy progresses, female embryos are more likely to be lost, especially during a critical window between weeks 10 and 15.

According to the findings published in the Proceedings of the National Academy of Sciences:

“Data from around 800,000 amniocentesis tests showed that between weeks 10 and 15, significantly more female embryos are miscarried.”

That’s the twist.

Yes, male embryos may be slightly more prone to abnormalities in the first few weeks.

And yes, third-trimester mortality is still slightly higher for males.

But these smaller vulnerabilities don’t make up for the greater losses of females during the second trimester.

So even though boys have a rougher time later in gestation, the greater number of female deaths earlier on is what ultimately skews the birth ratio.

The fragile male? He might just be a myth.

Why Are Female Embryos More Likely to Die?

This leads to the next big question: why are girls more vulnerable in the womb?

The answer, at this point, is far from settled. But researchers have some leads.

One possible explanation has to do with the X chromosome.

Unlike males (who have one X and one Y chromosome), females have two X chromosomes—one from each parent.

This redundancy can offer protective benefits against some diseases, but it may also introduce complexities during early development.

In particular, the research team speculates that the paternal X chromosome may actually slow down development in a way that increases the risk of miscarriage.

“One possible mechanism is that a paternal X chromosome retards development in such a way that female mortality rate increases; this has been confirmed in the mouse,” the team reports.

In mice, similar developmental lags have been linked to higher female embryo mortality, so it’s possible that something similar is at play in humans.

But until further research is done, it remains a compelling—but unconfirmed—hypothesis.

The Evolutionary Puzzle of Miscarriage

This entire discovery adds a new layer of complexity to the already murky science of miscarriage—a phenomenon that affects up to one in four known pregnancies and likely many more unknown ones.

Understanding when and why embryos die could have major implications for:

Improving IVF success rates
Predicting and preventing miscarriage
Revealing hidden genetic vulnerabilities

And yet, it’s only recently that we’ve had the tools to track embryo development from day one with the kind of precision needed to uncover these patterns.

“No one really sat down and said ‘let’s try and make sense of all this,'” Orzack said. “It gets in textbooks and then it’s viral.”

What’s become clear is that our previous assumptions may have missed the mark, not because scientists weren’t asking questions, but because the data was fragmented, scattered across studies, clinics, and diagnostic labs.

This research unified it all.

And in doing so, revealed that biology isn’t playing favorites the way we thought.

So Why Are More Boys Still Born?

Let’s recap.

At conception, boys and girls are created equally.
During early pregnancy, some males are lost to developmental abnormalities.
Between weeks 10–15, more females are lost due to unknown vulnerabilities.
In the third trimester, slightly more males are lost again.

But in total? More girls die.

That explains the final birth ratios: 103 to 107 boys for every 100 girls. It’s not that more boys are made—it’s that more girls don’t survive.

This subtle, invisible war within the womb has been hiding in plain sight, masked by assumptions, small studies, and biological noise. Until now.

How This Impacts Medicine and Miscarriage Research

What can we do with this information?

First, it shifts how we understand fetal viability and vulnerability.

Rather than assuming a uniform risk of miscarriage or developmental issues, we now have to consider sex-specific “windows of vulnerability”.

Early pregnancy loss in males may signal chromosomal defects.
Second trimester loss in females may point to subtler developmental challenges tied to X chromosome interactions.

This might allow physicians to:

Develop better screening protocols
Predict risk factors more accurately
Tailor interventions based on fetal sex and gestational age

It may also explain why some women experience recurrent miscarriages without clear cause—especially if the losses skew female and occur during the second trimester.

More broadly, it’s a reminder of how nuanced and asymmetric human development really is.

Unpacking the Broader Implications

Let’s zoom out even further.

When sex ratios skew naturally at birth, it may seem like a biological footnote.

But in the context of assisted reproductive technologies, prenatal screening, and even societal preferences for one sex over another, small natural skews can have outsized cultural and medical consequences.

In countries where sex-selective abortion has been a concern, understanding the natural ebb and flow of male and female fetal development becomes even more critical.

If you don’t understand when and why embryos are lost, it’s easy to misinterpret gender ratios at birth.

This study forces us to reconsider long-held ideas—not just about biology, but about how we interpret pregnancy outcomes, health risks, and gender expectations.

And it shows just how much we don’t yet know about life in the womb.

What We Think We Know vs. What the Data Shows

For decades, we’ve told ourselves that the male embryo is the fragile one.

That girls are biologically hardier, more robust, more likely to make it.

This belief shaped research, medical care, and public understanding of reproduction.

But Orzack’s work reminds us that science is a process of correction, not certainty. Ideas that get into textbooks are often decades ahead of the evidence.

And sometimes, they’re just wrong.

So, the next time someone casually tells you that “boys are more likely to die before birth,” feel free to share this story.

Tell them about the 140,000 pregnancies.

The 30 million data points.

The researchers who finally connected the dots.

And tell them this: