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What Decides The Sex Of A Baby?

A baby asleep

Image Credit: Peter Oslanec

The big question that all parents want to know when they discover that they are expecting a baby is, what sex will the baby be? Will it be a boy or a girl?

Many people wonder if they can influence the sex of their baby.

Is it possible to choose the sex of your baby?

What Determines The Sex Of A Baby

The sex of a baby is determined by its chromosomes. Chromosomes are a structure of nucleic acids and protein which are found in the nucleus of most living cells. They carry genetic information in the form of genes.

In humans and mammals, the sex chromosomes are called X and Y. The egg that forms in a female’s body always has an X chromosome. The sperm produced by the male may have either an X or a Y chromosome.

What determines the sex of your baby.

When a sperm fertilises an egg, it passes on its genetic material to the egg.

So if the sperm has a Y chromosome, a male baby with an X and a Y chromosome will be formed.

If the sperm has an X chromosome, a female baby with two X chromosomes will be formed.

There are a number of very rare chromosomal abnormalities which may occur where they may have extra chromosomes, which may include XXX, XXY and XYY which are the most common, and there are others which are even rarer.

One a sperm has fertilised an egg, there is no way of changing the sex of that baby.

Can I Influence The Sex Of My Baby?           

It is always the man’s sperm that determines the sex of any baby. The woman has no influence over the sex of her baby.

It is the genetic material in the man’s sperm is the only thing that affects the gender of any baby that is born.

So while it is fun to use gender predictors such as the Chinese Gender Predictor, the Mayan Gender Predictor or one of the many other old wives’ tales about guessing the gender of your baby, the answer is this.


You cannot influence the gender of your child. Nature will do whatever it wants.

It will make no difference when you have sex, how old you are, what position that you use, where the stars are aligned or what you eat before conception.

Is There Any Way To Choose The Gender Of My Baby?

There are currently two scientific techniques available for choosing the sex of your baby.

In the UK it is illegal to use either of these techniques to choose the sex of your baby unless they are carried out to avoid genetic conditions which can be passed on by a particular sex. Your eligibility for this treatment would have to be decided by the Human Fertilisation and Embryology Authority, so treatment cannot take place without the authority’s consent.

It is also heavily regulated in every country in the world where the techniques are available. In every country it is also only allowed where it used for minimising the risk of passing on genetic disorders.

The two techniques are:-

  1. Pre-implantation Genetic Diagnosis (PGD)
  2. In this technique, embryos would be created by In Vitro Fertilisation (IVF), and only embryos of the sex that is wanted will be implanted.

  3. Sperm Sorting
  4. In this method, the man’s sperm is sorted into those carrying an X and those carrying a Y chromosome. The mother is then inseminated with the sperm of the desired gender by artificial insemination.

Both these techniques are also extremely expensive, and some are banned outright in some parts of the world.  And even then, there is no guarantee that you will have the baby of the sex that you hoped for; these techniques merely increase the probability that you will have a baby of any particular gender.

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