What are the Chances of Having a Boy vs Girl

Published on March 19th, 2021

Pregnant woman holding a gender reveal card

It’s generally understood that the chances of having a boy or a girl boil down to 50/50 odds—essentially a coin toss. Heads for boys, tails for girls!

But are the chances of having a boy or girl really as simple as a flip of the coin?

Nope! They’re actually much more interesting. 

Not Quite 50/50: Understanding the Odds of Having a Boy or a Girl 

According to the World Health Organization, for every 105 boys born globally, there are about 100 girls born. This is called the sex ratio at birth. Sex ratio at birth is a term used in studies to describe the ratio of boys to girls (always in that order) in a population when they’re born. 

So really, that coinflip is a little bit more weighted in favor of having a baby boy. 

  • What are the chances of having a boy? – If you’ve got your fingers crossed for a sweet baby boy, the odds are just ever so slightly in your favor. You have about a 51% chance of having a boy.
  • What are the chances of having a girl? – If you’re hoping for a bright-eyed baby girl, your chances are still very good at around 49%.

However, these numbers are based on a global sex ratio at birth—it doesn’t account for the many variables that could specifically impact your chances of having a boy or a girl.

If you’re wondering, “Can you pick the gender of your baby?”, the answer is generally no. However, factors such as paternal influence from a physiological perspective (more on this later), times of crisis, parental age, and geographic location can sway whether you will be having a sweet little girl or an adorable little boy.

Factors that Affect Sex Ratio at Birth 

The 50/50 chance is a “close enough” metric when you’re speaking in generalities about the global sex ratio at birth. However, scientists, anthropologists, and sociologists are finding compelling data that the ratio can be swayed in one direction or the other in specific locations or instances.

Paternal Influence 

Dad’s genetic contributions are responsible for plenty about your little bundle of joy to-be, including one of your child’s most foundational characteristics—gender.

  • The background – On a molecular level, a baby starts as two different reproductive cells—an egg cell contributed by Mom and a sperm cell contributed by Dad. During conception, these reproductive cells fuse together and create an embryo, the cellular start of your future child.

Each cell brings its own unique set of genetic material to the table. When it comes to the baby’s gender, a male sperm cell can carry either a Y or an X chromosome, while the egg always carries an X chromosome. When a Y-carrying sperm meets an egg, they create a male embryo. When an X-carrying sperm meets an egg, they create a female embryo.

  • Ratio of Y-sperm to X-sperm – Yup, Dad’s cells are the ones that determine the gender of your child (but don’t worry, a baby gets plenty of unique traits from Mom). One thing that could tip the balance of the 50/50 ratio? If Dad doesn’t create equal amounts of Y-carrying sperm and X-carrying sperm.

Scientists are examining whether certain genes in a man’s DNA impact the Y/X ratio of his sperm. DNA is like your body’s technical manual—it tells cells how to make all the parts that make you you. So it’s possible that under the “Sperm Production” section of that manual, there are clear instructions for the sperm-making systems of a dad’s body to always produce a 3:1 ratio in favor of Y-carrying sperm, increasing your chances for a male child substantially.

However, no specific gene has been isolated (yet). But scientists do have another way to examine the paternal genetic predisposition for having boys or girls—family trees.

  • A genetic link through siblings Researchers in England conducted a study to determine whether there was more to Dad’s genetic contribution to a baby’s sex. They looked at over 927 family trees dating back to 1600. They discovered that the sex ratio tended to follow the dad’s side of the family.

For example, if a dad had more brothers, he was more likely to have boys himself. But, if a dad had more sisters, then his children were more likely to be girls.

More studies need to be done to understand Dad’s genetic role in your chances of having a boy or a girl, but if you’re wondering how to have a baby boy, a good look at your partner’s family tree couldn’t hurt! 

Times of Crisis

We all face little crises every day, from hitting “reply all” to a company email to forgetting a parent’s anniversary. These are typical issues we conquer daily and don’t create lasting biological impacts.

But what about big crises? Ones that drastically impact day-to-day life for an entire population?

Multiple studies show that war, famine, and environmental phenomenon like earthquakes impact sex ratio at birth—in favor of girls.

  • Iran-Iraq War, 1980 Prior to the Iran-Iraq war, the Iranian sex ratio at birth was in line with the global ratio, 105 boys for every 100 girls. However, during the first year of the Iran-Iraq War, the ratio dropped significantly. Instead of an extra 5 male births, the ratio decreased down to an even 50/50.
  • Zakynthos Earthquakes, 2006 In 2005, the sex ratio on the island of Zakynthos was 1.1:1. So for every 11 boys born on the island, 10 girls were born. But in 2006, moderate earthquakes shook Zakynthos, an island of Greece, for an entire month. Researchers found that in the year after the month that shook the world, the sex ratio decreased 50% from the previous year. Instead of 11 boys for every 10 girls, only 6 boys were born for every 10 girls. 

Why do times of crisis have such a big impact on the gender ratio? History, psychology, and biology offer an answer:

  • Stress and gender determination Scientists have recently found a connection between a mom-to-be’s stress levels and the likelihood of having a boy or a girl baby. Biologists discovered that moms who are carrying boys eat highercalorie diets. But during times of crisis, there may not be enough calories to go around.

Could there be an element of self-preservation at play during conception? Scientists have found evidence that the woman’s egg plays a more active role in the fertilization process, even suggesting that eggs may have sway on which sperm becomes part of the embryo. So in times of crisis, could the egg choose a female sperm that will conserve resources for Mom in the long run? Maybe. Researchers certainly have a lot more research to do when it comes to how to have a baby girl! 

Paternal Age

We’ve all heard about the “biological clock.” You know, the one that tells you hey, it’s quarter past baby-making o’clock! But when you decide to heed the call of the clock may have an impact on your chances of having a son or a daughter. 

One study in Denmark sought to help illuminate that not-quite 50:50 ratio of boys to girls. They studied the births of children between 1980 and 1993, over 800,000 babies in all. They examined multiple births, gender ratios of siblings, birth order, and location. One of the most interesting connections they found was between paternal age and a child’s sex:

  • Younger dads were more likely to have boys. 
  • Older dads were more likely to have girls.

Fathers who were younger than 25 years old were often in line with the birth ratio of the country, with a 51.6% chance of having boys. However, fathers over 40 had just a 51% chance of having a boy.

The decrease is slight, but half a percentage point can make a big difference in a population. However, further studies on how age impacts the sex-at-birth ratio are needed before we can say how old you are makes a difference in your chances of having a boy or a girl. 

Geographic Location 

Where you live in the world may affect your chances of choosing blue paint or pink for your baby room. For example, in Azerbaijan, the sex ratio is 111 boys born for every 100 girls, but in Zimbabwe, it’s only 102 boys born for every 100 girls!

Why such a difference? All the reasons we listed above can come together to impact a nation’s sex ratio at birth. For instance:

  • Countries that have been through recent wars may experience a decrease in their sex ratio, in favor of girls.
  • Nations where parents start having children at younger ages may have a higher sex ratio, in favor of boys.

After all, a country is its people. A multitude of demographic, environmental, and cultural factors may come together to influence the odds of having a boy or a girl in unique ways. 

Girl Or Boy? Find Out Sooner with SneakPeek! 

Even if your partner is the only boy in a family full of girls, or you’re living through something stressful like… a global pandemic, it’s hard to know precisely how that gender coin-toss will land for your child. 

The good news? You won’t have to wait too long to find out, thanks to the SneakPeek Early Gender DNA Test. 

The SneakPeek Early Gender DNA test lets you discover your child’s gender as early as 8 weeks into pregnancy. Our simple DNA gender prediction test comes with easy-to-follow instructions and prepaid packaging so you can test, ship, and find out your baby’s gender seamlessly.

Find your answers sooner than ever before, with SneakPeek!


The Bump. https://www.thebump.com/a/more-baby-boys-than-girls

Psychology Today. https://www.psychologytoday.com/us/blog/valley-girl-brain/202003/what-are-your-chances-having-boy-or-girl

Pew Research Center. The odds that you will give birth to a boy or girl depend on where in the world you live. https://www.pewresearch.org/fact-tank/2013/09/24/the-odds-that-you-will-give-birth-to-a-boy-or-girl-depend-on-where-in-the-world-you-live/

World Health Organization. Sex Ratio. https://www.who.int/data/maternal-newborn-child-adolescent-ageing/indicator-explorer-new/mca/sex-ratio-at-birth-(male-births-per-female-births)

Springer. Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene. https://link.springer.com/article/10.1007/s11692-008-9046-3

Proceedings of the National Academy of Science, USA. Maternal prenatal stress phenotypes associate with fetal neurodevelopment and birth outcomes. https://www.pnas.org/content/116/48/23996

BMJ. Changing sex ratio in Iran, 1976–2000. https://jech.bmj.com/content/56/8/622

Royal Society Publishing. You are what your mother eats: evidence for maternal preconception diet influencing foetal sex in humans. https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2008.0105

Oxford Academic. Natural variation in the human sex ratio. https://academic.oup.com/humrep/article/14/12/3120/2913108

Phys.org. Fertilization discovery reveals new role for the egg. https://phys.org/news/2019-11-fertilization-discovery-reveals-role-egg.html

SneakPeek aims to provide the most accurate and up-to-date information to help our readers make informed decisions regarding their health before, during, and after pregnancy. This article was written based upon trusted scientific research studies and/or articles. Credible information sources for this article are cited and hyperlinked.

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