At What Point is a Baby's Sex Determined?

Whether you want to find out your baby’s sex or not, you may be interested to know the ins and outs of how sex is determined. We’ve covered everything from what (and who) determines the sex of a baby, in which week a baby’s sex is developed, and at what point in your pregnancy you can find out the sex.

What Determines the Sex of a Baby?

Are you ready to walk down memory lane and channel your high school biology class? You might remember that human body cells have a total of 46 chromosomes (23 pairs), making up our genetic material. Your baby starts off with just one cell: the combination of an egg and a sperm. But when it comes to what and who determines the biological sex of a baby, only two of those chromosomes in that initial cell play a role.

At the moment of fertilisation, one chromosome from the sperm and one from the egg determine the sex of the baby. These are aptly known as the sex chromosomes, and every egg has two X chromosomes while each sperm has either an X or a Y chromosome to determine the sex.

So, when does an embryo become male or female, and does the sperm determine the sex? It’s up to chance and depends on which sperm reaches the egg first and fertilises it:

  • If the winning sperm has an X, the baby will be female (XX)

  • If a sperm with a Y chromosome reaches the egg first, the baby will be male (XY).


Who Determines the Sex of a Baby?

Perhaps you’ve wondered does the egg or sperm determine the sex of your baby? It takes two to tango, but scientifically, it only takes the sperm to determine the baby’s sex. The egg will always have an X chromosome, so the sperm will be the decider.

At the point of ejaculation, around 150 million sperm are released, which then attempt to move through the vagina, cervix and uterus to reach the fallopian tubes. For a microscopic sperm, this is quite a long journey, and many are lost along the way. Out of that original number, only a few dozen will attempt to penetrate the egg, and whichever does it successfully will create a little zygote with the egg.

Of course, it’s all chance, as in which sperm reaches the egg first and successfully penetrates it, so when asking who determines the sex of the baby, it’s less about who does it and more about how it happens.

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Common Questions About Baby Sex Determination

Though baby sex determination is relatively straightforward from a scientific standpoint, there are many questions that soon-to-be parents commonly ask regarding what and who determines the sex of a baby, and we’re here to help answer them.

  1. Do all babies start as female? No, but we can understand why this is a common question, as all eggs in a female body have an X chromosome. You won’t have determined biological sex until the egg’s X chromosome comes together with a sperm’s X or Y chromosome.

  2. Does the female egg or male sperm determine sex? Whichever chromosome the sperm that fertilises the egg has is what determines whether a baby will have male or female sex chromosomes.

  3. What sex do you start as in the womb? Again, baby sex determination starts at fertilisation when the egg and sperm meet. So, when you were conceived, your sex while in the womb depended on which chromosome the sperm had – X for a girl and Y for a boy. The same goes for your baby!

  4. Does family history determine a baby’s sex? It could be that family history plays a role in some cases, though this hasn't been conclusively demonstrated. However, genetics definitely does impact whether you might have a non-identical twin pregnancy!

In Which Week is a Baby’s Sex Developed?

You may be interested in when and how you can tell the sex of your baby, especially if you’re planning what’s commonly known as a ‘gender reveal’ party. If this is something you're considering, check out these gender reveal ideas that can help make your celebration a memorable one!

Though fertilisation is when your baby’s biological sex is determined, you may not be able to know this piece of information until about halfway through your pregnancy. If you wish to know whether you’re having a boy or girl, inform your sonographer or midwife before your 20-week scan. Because finding out the sex of your baby is not offered as part of the national screening programme, you may have to check your hospital’s policy to find out if they offer this service.

The midpoint of pregnancy is when boy or girl’s genitalia parts may be visible enough on an ultrasound to decide. Sex and foetal genitalia development is influenced by your baby’s sex chromosomes with females having two X chromosomes and males having an X and Y chromosome.

Can the Sex of Baby Change During Pregnancy?

Sex determination of a baby happens during fertilisation, and it can’t change during your pregnancy. It’s also very rare for the sonographer to read the 20-week ultrasound incorrectly and tell you you’re having a girl (or a boy) when the opposite is true.

If you want to have a little fun while you wait for the big moment, check out some fun ways to ‘predict’ your baby’s sex. And maybe your pregnancy cravings will give it away!

When Does an Embryo Become a Foetus?

It’s common for parents to wonder about the terminology of their baby during pregnancy, for example, when does an embryo become a foetus? Until week 8 of pregnancy, your little one is referred to as an embryo and at 8 weeks they are called a foetus, until birth.

Chances of Having a Girl vs. a Boy

Why do I keep having baby girls, or why do I only have baby boys? You aren’t the only parent asking yourself these questions!

Again, we know that the sperm’s chromosome is what determines the sex of a baby. If we assume that the male produces the same amount of X-chromosome sperm as Y-chromosome sperm, probability dictates that the chance of having a girl would be the same as the chance of having a boy. Most experts concur that the chances of having a girl vs. boy are around 50:50. And this goes for each time you have a child, so the odds of having a girl after a boy would still be about 50:50.


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Can Babies Be Born With Both Sexes?

A baby could be born with what’s called ambiguous genitalia, which could mean:

  • Incomplete genitals. Some babies born with ambiguous genitalia have genitals that haven’t developed fully, so they lack characteristics of either sex.

  • Both genitals. Other babies with ambiguous genitalia either have characteristics of both sexes or have external genitalia that doesn’t match the internal sex organs.

Another name for ambiguous genitalia is intersex or differences of sex development (DSDs). A baby born with incomplete genitals, characteristics of both sexes or ambiguous genitalia, may be detectable shortly after birth. In these cases, your doctor or specialist nurse will be your go-to support, offering information about whether treatment is necessary.

The Difference Between a Baby’s Sex and Gender

When people talk about a baby's sex and gender, they often use the terms interchangeably. However, there’s actually a difference between the two. Let's clear things up!

Sex refers to the biological characteristics that determine whether someone is male or female, such as their reproductive organs and chromosomes. Gender, on the other hand, refers to the social and cultural roles, behaviours and expectations that are associated with being male or female. In other words, sex is what you're born with, while gender is how you identify and express yourself.

It's important to recognise that not everyone fits neatly into traditional binary categories of male or female assigned to them when they are born – gender identity is fluid, personal and can vary from person to person. Ultimately, it's up to each individual to define their own gender identity in a way that feels authentic and true to themselves at a time that is right for them, or, as they become more aware of how they identify vs the physical anatomy of their body.


Neither biological parent is fully responsible for determining the sex of the baby. However, all eggs produced by the female have an X sex chromosome, and all sperm from the male have either an X or Y chromosome. If an X sperm fertilises the egg first, you’ll have a girl (XX), and if a Y sperm fertilises the egg, you get a boy (XY).

The Bottom Line

What and who determines the sex of a baby? It all boils down to good old-fashioned science, along with chance, and happens very early on during the fertilisation process. The X and Y sex chromosomes in the egg and sperm are what determine the biological sex of your little one. Eggs always have an X chromosome, and sperm have either an X or Y chromosome. Whichever sperm is the winner will penetrate and fertilise the egg, creating either an XX (girl) or XY (boy) pair.

Though your baby’s sex is determined at fertilisation, foetal genitalia development won’t be apparent visually until later in your pregnancy. You’ll need to wait for your mid-pregnancy ultrasound before learning if you’re having a boy or girl, generally a 50:50 chance.

How We Wrote This Article The information in this article is based on the expert advice found in trusted medical and government sources, such as the National Health Service (NHS). You can find a full list of sources used for this article below. The content on this page should not replace professional medical advice. Always consult medical professionals for full diagnosis and treatment.

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