What Determines the Gender of a Baby?

Published on August 27th, 2020 and Updated on April 1st, 2021

For millennia, scientists and astrologers have attempted to crack the mystery of predicting a baby’s sex. Stars were consulted, charts were made, but most gender prediction methods were correct 50% of the time—no better than a coin flip.

What answers were our ancestors not privy to? What determines the gender of a baby? Put away that telescope— the stars can’t tell you. A microscope might be a better tool for this discovery. Continue to learn more about how to determine the gender of a baby as well as how to use a gender prediction test to do so. 

How A Baby’s Gender is Determined: A Cheat Sheet to Unlocking the Gender Mystery 

We first begin with how a baby is made. We’re referring, of course, to the science of baby-making—not the mechanics. 

Let’s start with some reproductive terminology to help paint a picture of what happens on a cellular level. 

  • Gamete – A gamete is a reproductive cell. Your body is made of a variety of different cells, like muscle cells, immune cells, and brain cells. Each cell has a specific function to help support your body’s health. A gamete’s job is to store your genetic information and use it to create a new life—a baby. There is one type of gamete for each of the two genders – sperm for males and eggs for females.
  • Sperm – Sperm is the male reproductive cell—also referred to as the male gamete. It holds the father’s DNA and fertilizes the egg to create an embryo, the cluster of cells that grow into a baby.
  • Egg – Female gamete, ovum, egg—this cell has many names. Essentially, the egg is a woman’s reproductive cell. It stores the mother’s DNA and is found in the lining of the uterus.
  • Embryo – Unlike a gamete, an embryo isn’t a single cell, but instead, the combination of two cells—a male gamete and a female gamete. When a sperm fertilizes an egg, together, they create an embryo, the cluster of cells that will become a baby.
  • Chromosome – The egg and the sperm both carry genetic information in the form of chromosomes. When an egg is fertilized, the male sperm provides half of an embryo’s chromosomes while the egg provides the other half to make a complete set of genetic information—enough to create a new human life.

How does gender fit into this picture? 

Chromosomes determine everything from a baby’s eye color to shoe size. The pair of chromosomes that determine a baby’s gender are called sex chromosomes. There are two kinds of sex chromosomes, X or Y.

Women have two X chromosomes while men have one of each kind, an X and Y. To complete the chromosome pair, a mom-to-be’s egg provides one sex chromosome—always an X chromosome—and the sperm provides the other—which has the potential to be either an X or a Y chromosome. 

Eggs can only provide an X chromosome because the woman is the source of an egg’s genetic material and women only have X chromosomes. But men have XY chromosomes—that means that a sperm can bring either an X or a Y chromosome to the egg.

If a male sperm carrying an X chromosome fertilizes the egg, it will combine with mom’s X, and the embryo will have two X chromosomes (XX) and become a girl. If a Y-carrying sperm combines with mom’s X, the resulting embryo will have one X and one Y chromosome (XY) and grow up to be a boy. 

Chromosome Fun Fact: Because some genetic diseases are only found on either the X or Y chromosome, there are interesting cases of inherited diseases. For example, if a genetic disease is on the X chromosome, women can carry the disease but not express it because their other X chromosome (which doesn’t have the disease expression) can be dominant. Men, on the other hand, only have one X chromosome, meaning if the disease expression is there, it will show up. Hemophilia and red-green color blindness are two examples of an X chromosome-specific disease. As a result, 95% of red-green color blind cases are male.

So, what do these gamete cells say about who determines the gender of a baby?

Gender is Determined by the Father 

Because sperm cells can carry either of the two sex chromosomes, it’s the male who technically determines the baby’s gender. This raises another interesting question:

Are sperm cells equally likely to contain an X chromosome as they are to contain a Y chromosome?

Thanks to current medical research, it looks like the answer is no

Evidence 1: World Bank Birth Rate Data 

Pew Research investigated the World Bank birth rate data to determine the global sex ratios of males to females.

Historically, they found that the male-to-female birth ratio was around 1.05. Meaning there have been 105 male births for every 100 female births. What’s interesting is that even in countries with the lowest male-to-female birth ratios still show a disproportionate share of boys to girls.

It is more likely for a Y chromosome sperm to fuse with an ova cell and a boy to be born. This suggests that either there are more Y chromosome sperm cells or that these sperm have a slight edge in the “swimming competition.”

Evidence 2: The Family Tree Study 

In a large genealogical study, Newcastle University researchers analyzed hundreds of family trees that were documented in a genealogy forum. Some of these trees dated back to 1600, and altogether they contained information on over half a million people!

When it came to the question of whether males were more likely to have sons or daughters (i.e., more likely to have more Y chromosome sperm vs X chromosome sperm), they observed:

“The family tree study showed that whether you’re likely to have a boy or a girl is inherited. We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. However, in women, you just can’t predict it.”

The researchers speculate that the creation of sperm cells is likely to be determined by two types of genes. In a simplified scenario:

  • If a male has 2 “m” genes, he is more likely to produce Y chromosome sperm and have sons.
  • If a male has 1 “m” and 1 “f” gene, he is equally likely to produce either X or Y chromosome sperm.
  • If a male has 2 “f” genes, he is more likely to produce X chromosome sperm and have daughters.

Evidence 3: War Time Births 

Another factor of whether males produce X or Y chromosome sperm may have to do with the height of the male, as evidenced during war time.

It’s been documented through historical records that during and after war times, countries will experience more male births. Is this mother nature’s way of rebalancing the population after a large number of males die during combat? Or is something else going on?

After World War I, the heights of soldiers returning from combat were compared with those who didn’t survive. Turns out, the average height of returning soldiers was nearly an inch taller than those who had fallen (about 66.4 inches to 65.5 inches). Satoshi Kanazawa says evidence purports taller parents are more likely to have sons than shorter parents. This small 1-inch difference across a generation of soldier-parents is enough to explain millions of excess boy births than girl births in the UK post WWI.

Are there different likelihoods of having a boy or girl? 

To summarize, yes, there are different likelihoods of having a boy or a girl. And while the chances may be close to 50-50, evidence suggests that you can look toward the male’s family history and height for some indications as to which direction that chance skews.

The one way to know for sure? An early gender DNA test.

SneakPeek Early Gender DNA Test 

By 8 weeks into pregnancy, women will have tiny pieces of DNA from their growing baby in their bloodstream—this DNA is called cell-free fetal DNA. A simple gender blood test can screen this cell-free fetal DNA and look for Y chromosomes. Because mom has two X chromosomes, if the test detects any Y chromosomes, then that means she’s having a boy! If no Y chromosomes are detected, then she’s having a little girl! 

This is how the SneakPeek Early Gender DNA Test works.

Wondering how to know baby gender without an ultrasound scan? If you’re interested in discovering your baby’s sex sooner than ever before, you can take the test from the comfort of your home as early as 8 weeks into pregnancy.

The test kit will arrive with clear step-by-step instructions and everything you need to take the test. Once you’ve completed the test, you can mail your DNA sample to SneakPeek Labs using the prepaid packaging and receive the exciting news soon after the sample is received.

It’s simple, safe, and best of all, accurate. The Early Gender DNA Test has proven to be 99.9% accurate in laboratory studies.

Are you ready to know what your little one’s 23rd pair of chromosomes reveal?



Palomar.edu Recombination and Linkage. https://www2.palomar.edu/anthro/biobasis/bio_3.htm

University of Rochester Medical Center. X-linked Recessive: Red-Green Color Blindness, Hemophilia A. https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=90&ContentID=P02164

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/

ScienceDaily. Boy or girl? It’s in the father’s genes. https://www.sciencedaily.com/releases/2008/12/081211121835.htm

Psychology Today. The Returning Soldier Effect I: Why Are More Boys Born During and After Wars? https://www.psychologytoday.com/us/blog/the-scientific-fundamentalist/200802/the-returning-soldier-effect-i-why-are-more-boys-born

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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