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How to Conceive a Boy or a Girl (According to Science)

Is there a way to predetermine your baby's biological sex and conceive a girl or a boy? Here's what science has to say about things like genetics, timing, diet, and even adrenaline.

By
Sabrina Stierwalt, PhD
6-minute read
Episode #395
The Quick And Dirty
  • There is evidence that men may inherit the tendency to conceive more male or more female babies. 
  • There is evidence that a woman who eats a diet higher in calories may be more likely to conceive a biological boy.
  • Scientific studies have not been able to support claims that the timing of sexual intercourse can influence whether a baby is born a biological boy or girl.

As someone who never found out the sex of my babies until they joined us on the outside, I’m not sure I fully understand our fascination with the sex of unborn babies. We even have gender reveal parties (which—ahem—should be assigned-sex-reveal parties) so elaborate that they cause acres of wildfire damage.

Total strangers would stop me on the street to say things like 'Oh wow, you're carrying low; that's definitely a boy.'

During my pregnancies, total strangers would stop me on the street to say things like “Oh wow, you're carrying low; that's definitely a boy.” Hmm. I’m pretty sure the low-hanging belly just means I had no ab muscles to speak of when we started on this pregnancy journey, so the muscles that were there just gave up.

But what does fascinate me is whether or not there's any scientific evidence to back up the common lore surrounding how women might influence the sex of their baby. I’ve heard many times that eating a lot of salty snacks and red meat while you’re trying to conceive will produce a boy. Or, if you have a lot of chocolate and yogurt in your diet when you get pregnant—and who doesn’t?—you’re destined to have a girl.

Sometimes advice like this gets passed along without any backing, but sometimes it’s based on convincing anecdotal data. 

How is a baby's sex determined?

The sex of a fetus is determined by its X and Y sex chromosomes. Very simply put, the provider of the egg typically also provides an X chromosome while the sperm can either contribute another X chromosome or a Y chromosome. Two X chromosomes (XX) make a girl, and an X and Y (XY) chromosome combined make a boy. There are rare instances when a sperm may have both an XY which creates a boy with Klinefelter syndrome (an XXY boy) or no sex chromosome to create a girl with Turner syndrome (an X girl). 

Two X chromosomes (XX) make a girl, and an X and Y (XY) chromosome combined make a boy.

There are ways to use technology to essentially choose the sex of the baby. An embryo with a known sex can be implanted through in vitro fertilization (IVF) via a process called pre-implantation genetic diagnosis. 

Sperm can also be sorted into X and Y carriers before being used in artificial insemination. One such sperm sorting technology, called MicroSort, uses the fact that sperm carrying an X chromosome are bigger. Scientists first cover healthy sperm with a fluorescent stain and then shine lasers on them to excite that fluorescence. The sperm cells that glow brighter are bigger and thus more likely to be carrying an X chromosome. MicroSort is currently only available in Mexico, Switzerland, and Northern Cyprus.

Can we influence the biological sex of an unborn baby?

Humans are clearly tinkerers, and tinkering with biological sex, despite the possible ethical dilemmas associated with such technology, is clearly no exception. But what about more natural methods? Does a pregnant person really have a 50-50 chance of having a boy or a girl? 

Men can inherit the tendency to produce boys or girls

Authors of one study published in the journal Evolutionary Biology combed through 927 family trees spanning back centuries. That’s over 550,000 people. They noticed a clear pattern in the data: Men could inherit the tendency to have boys or the tendency to have girls. In other words, if a man has more brothers, he may be more likely to conceive sons. The same was not true for women—there was no clear connection that a woman with many sisters, for example, would then have more daughters.

There may be an as-of-yet undiscovered gene that dictates a person’s tendency to have boys or girls.

The geneticists conducting the study were not able to confirm the biological reason for this connection. But they ran simulations of genetic models that showed there could be a hereditary factor linked to the types of alleles that sperm carry. Those alleles are responsible for controlling the ratio of X-carrying to Y-carrying sperm. So there may be an as-of-yet undiscovered gene that dictates a person’s tendency to have boys or girls. 

Since this study looked back to the 1600s, the family trees also spanned a huge range of human experience. The study authors noticed another pattern: the ratio of biological boys to biological girls increases after times of war. Other studies have come up with similar results. For example, in the year after World War I ended, an extra two biological boys were born for every 100 girls in the UK. 

Some scientists think these two patterns are connected. Since men are traditionally those who fight in wars, a family with more sons is more likely to have at least one son return safely. When that survivor, a man with more brothers, goes on to have children, he is then more likely to have sons. 

How The World Cup may have affected the sex of babies 9 months later

On the other end of the human experience, studies have also shown that levels of adrenaline during conception may affect biological sex. In February and March of 2011 in South Africa, for example, 50.6% of the babies born were male as opposed to 49.4% female. That may not seem like a huge difference, but the ratio was the largest it had been in a decade.

This baby boy boom suggests that a combination of the frequency of sex, positive (read: unstressed) feelings, and adrenaline may all contribute to sperm motility.

So, what was happening nine months before when these babies were being conceived? The World Cup was being played in South Africa! The authors of the study that noted this baby boy boom suggest that a combination of the frequency of sex, the positive (read: unstressed) feelings of South Africa doing well in the tournament, and adrenaline may all contribute to sperm motility. Higher sperm motility gives those smaller Y-carrying sperm a better shot. 

A woman's diet may affect her baby's sex

Studies have also shown that women who eat more calories around the time of conception—something we tend to do when times are good and resources are plentiful—are more likely to conceive a boy. Specifically, it has been noted that women who have boys tend to have more potassium, calcium, and vitamins C, E, and B12 in their diet. So there may be a link to those particular ingredients or it may just reflect a larger variety and amount of food. 

One study of 740 British women grouped them into three categories: high caloric intake, average caloric intake, and low caloric intake. The study authors found that 56% of the women in the highest calorie group had biological boys compared to 45% in the lowest-calorie group. I also got a laugh out of that study referring to the human species having a “high maternal investment” in creating offspring, which I wholeheartedly endorse. 

The Shettles Method

If you're a person trying to conceive and you haven’t heard of the Shettles Method, you’ve probably still heard the associated advice. Followers of the method assume that male sperm are smaller, have shorter life spans, and swim faster. They claim that having sex immediately following ovulation (and using specific sexual positions) can give those Y carrying sperm a better chance at being the ones that result in implantation.

Despite decades of trying, scientists have not been able to produce significant evidence to back up whether the Shettles Method is effective.

Shettles published his work in the 1970s. But despite decades of trying, scientists have not been able to produce significant evidence to back up whether it's effective. For example, one smaller study published in the New England Journal of Medicine showed no link between the timing of sexual intercourse and sex of the baby for 192 pregnancies.

Our understanding of what determines whether a particular couple will conceive a biological girl or a biological boy is still incomplete. We don’t yet know how much is genetically predetermined versus how much luck of the draw is involved.

Perhaps that’s nature’s way of telling us to focus on something else—like being happy, healthy parents raising happy healthy babies—instead. 

Sources +

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.