Boy or Girl: How Genetics Determine Your Baby’s Gender

Are you curious about what exactly determines the gender of a baby? Wondering how complex the difference between a boy and girl really is? Need more information while considering your options for gender selection?

Sometimes we focus a lot on the differences between women and men, girls and boys. But have you ever wondered about the intricacies of science and biology that lead to these differences? What goes on in the early days of human development to produce a baby boy or a baby girl?

The experts that work in gender selection clinics know all the secrets, and their answers may surprise you:

Sex Chromosomes

Humans have a pair of sex chromosomes; it’s one pair out of the 23 that hold the entirety of our genetic code. Men have an X chromosome and a Y chromosome while women have two X chromosomes. This means, as you may have guessed, that Y chromosome is pretty important when it comes to the gender of a baby.

Reproductive Cells

Human sperm and eggs carry 23 chromosomes, half the genetic material it takes to create a human. No matter the woman, all her eggs carry the X chromosome. But only about 50% of sperm carry the X while the others carry a Y. When a sperm penetrates the egg, the presence or absence of the Y chromosome alone determines the future-baby’s gender.


You might be surprised that it all comes down to a single Y chromosome, but we can actually narrow things down to an even smaller scale! One single gene on the Y chromosome, one out of 200 on the Y chromosome alone, codes for gender. But the presence of that Y chromosome and that single gene influences the expression of a number of other genes; this ultimately creates several differences between boys and girls.

Sex Development

At around the seventh week of gestation, the genes a fetus has or doesn’t have begin to influence the production of hormones and the development of sex organs. If a Y chromosome is present, the production of testosterone will begin, and male genitalia will form. If not, female genitalia take shape.


In most typical pregnancies, the anatomical differences between a boy and a girl reveal themselves in an ultrasound at around 16 weeks, or the second trimester. Genetic testing can reveal gender even sooner, and if you opt for gender selection with gender selection clinics, you could know even sooner still!

DNA and genetics are amazing forces of life we’ve only just begun to scratch the surface of understanding. But with what we know so far, we are able to do incredible things for you and your family. If you have questions about genetics or gender selection clinics, give us a call today… we have the answers you seek!