Choosing Boy or Girl? The Coming Era of Baby Sex Selection

Can you choose whether you have a boy or a girl? It is a question that has fascinated prospective parents for generations, and it has inspired countless theories about everything from diet and sexual position to the precise timing of intercourse.

Modern fertility science has taught us a great deal about conception, but it has also shown why many traditional methods of trying for a boy or girl should be treated with caution. While fertility treatment can identify the sex of embryos in certain circumstances, there is no reliable natural method for choosing your baby’s sex.

In the UK, there is another important consideration. Using fertility treatment to select a baby’s sex simply because the parents would prefer a boy or girl is prohibited. Sex selection may only be used in specific medical circumstances, such as avoiding a serious inherited condition linked to sex.

So what can science actually tell us, what remains experimental, and where does UK law draw the line?

What Determines a Baby’s Sex?

In typical human reproduction, an egg contributes an X chromosome. A sperm contributes either an X or a Y chromosome. An embryo with XX chromosomes will typically develop as female, while an embryo with XY chromosomes will typically develop as male.

Which sperm fertilises the egg is normally a matter of chance. This is why, across large populations, the numbers of boys and girls born are relatively close.

For many years, however, theories have suggested that X-bearing and Y-bearing sperm behave differently enough for parents to influence which one reaches the egg.

One of the most persistent claims is that Y-bearing sperm are faster but more fragile, while X-bearing sperm are slower but survive for longer. This idea has been used to support advice about timing intercourse according to whether a couple wants a boy or a girl.

It sounds plausible, but it should not be presented as an established biological fact. Current evidence does not support using these supposed differences as a reliable way of selecting a baby’s sex.

Can Timing Intercourse Help You Choose?

Timing intercourse around ovulation can certainly be useful when you are trying to become pregnant. The fertile window covers the days leading up to ovulation and the day of ovulation itself, and identifying this period can help couples concentrate intercourse when conception is most likely.

That is very different from using ovulation timing to choose whether you conceive a boy or girl.

Professional fertility guidance does not support the claim that particular intercourse practices can reliably influence the sex of a baby. The American Society for Reproductive Medicine (ASRM), for example, states that there is no convincing evidence of a relationship between specific coital practices and infant sex.

In practical terms, knowing when you ovulate may improve your chances of becoming pregnant, but you should not rely on having intercourse on a particular day before or after ovulation as a way of choosing your baby’s sex.

You can read the ASRM guidance on optimising natural fertility for more information about the fertile window and timing intercourse.

What About Diet, Position and Other Natural Methods?

There is no shortage of advice online about how to conceive a boy or girl. Suggestions can include changing your diet, altering vaginal pH, choosing particular sexual positions, timing orgasm or following special conception calendars.

These methods are sometimes presented with impressive-sounding explanations, but there is no good evidence that they provide a dependable way of determining the sex of a baby.

This distinction is important because fertility tracking itself can be useful. Ovulation tests, cervical mucus monitoring and other fertility-awareness methods may help identify the fertile window and therefore help couples time intercourse when trying to conceive. They should not, however, be promoted as methods of sex selection.

For couples trying naturally, the most realistic expectation remains that the sex of the baby is largely outside their control.

Can Science Select a Baby’s Sex?

This is where the subject becomes more complicated.

Although natural methods cannot reliably determine whether a baby will be a boy or girl, reproductive medicine can obtain genetic information about embryos created through IVF. Researchers have also investigated ways of separating sperm carrying X and Y chromosomes.

These technologies are very different from timing intercourse at home. They involve laboratory procedures, fertility treatment or experimental techniques, and their availability and permitted uses vary between countries.

It is therefore misleading to move from saying that scientists can identify genetic differences to suggesting that parents can routinely choose their baby’s sex. The scientific, medical, ethical and legal questions all need to be considered separately.

What Did the Japanese Sperm Research Find?

One study that attracted considerable attention was published by researchers from Hiroshima University in 2019.

The researchers investigated differences between X-bearing and Y-bearing sperm in mice. They found that activating receptors called TLR7 and TLR8 affected the motility of X-bearing mouse sperm. This allowed the researchers to separate populations enriched with X- or Y-bearing sperm according to their movement.

When the selected sperm were subsequently used in laboratory reproduction, the researchers were able to produce strongly sex-skewed mouse embryos and offspring.

It was an intriguing scientific result, but there is an important qualification: the research was conducted in mice.

It did not establish a simple, proven treatment that prospective human parents can use to select a baby’s sex. Research findings in animal reproduction do not automatically translate into safe or effective fertility treatments for people.

The study is therefore best viewed as an interesting piece of reproductive research rather than evidence that a new method of human sex selection is readily available.

The original research can be read in PLOS Biology.

IVF and Pre-implantation Genetic Testing

A more established area of reproductive medicine is pre-implantation genetic testing, or PGT.

PGT involves testing embryos created through IVF before an embryo is transferred to the womb. Different forms of PGT are used for different purposes. For example, PGT-M can be used where there is a known risk of passing on a serious inherited condition caused by a particular gene.

PGT-M was previously known as pre-implantation genetic diagnosis, or PGD, so older articles may still use that term.

Because genetic testing can provide information about an embryo’s chromosomes, it can sometimes reveal its sex. However, being technically capable of obtaining that information does not mean it can legally be used to choose a boy or girl simply because that is what the parents would prefer.

In the UK, embryo testing is regulated by the Human Fertilisation and Embryology Authority (HFEA). You can read more about PGT-M and PGT-SR on the HFEA website.

Is Sex Selection Legal in the UK?

UK law draws an important distinction between selecting sex for social reasons and doing so to avoid serious inherited illness.

Sex selection can be permitted in specific medical circumstances where there is a risk of a serious inherited condition related to sex. Some genetic disorders predominantly or exclusively affect one sex, and embryo testing may therefore form part of treatment intended to prevent a serious condition being passed to a child.

The HFEA states that UK law prohibits selecting the sex of a child for reasons other than preventing serious inherited illness.

This means that a couple cannot go to a licensed UK fertility clinic and request a female embryo because they already have sons, for example, or request a male embryo simply because they would prefer a boy.

Rules are different in some countries, which is why prospective parents may encounter overseas clinics advertising “family balancing” or sex-selection services. Anyone considering fertility treatment abroad should carefully investigate the medical standards, regulation, legal position and implications of treatment in that country rather than assuming the same safeguards apply as in the UK.

For the UK position, see the HFEA guidance on sex selection.

The Ethical Debate Around Sex Selection

Even where sex selection is technically possible and legally available, it raises difficult ethical questions.

Supporters of greater reproductive choice may argue that parents should have considerable autonomy over decisions about their families. The idea of “family balancing”, where parents who already have children of one sex hope for a child of the other, can also seem very different from systematic discrimination against one sex.

Others are concerned about where such choices could lead. If a preference for sons or daughters is widespread within a society, individual decisions could collectively affect the balance between the sexes. There are also concerns that allowing selection for non-medical characteristics could encourage us to think about children in terms of desirable and undesirable traits.

Another difficult question is where a line should be drawn. If prospective parents could choose sex for non-medical reasons, should they eventually be allowed to select other characteristics if technology made that possible?

These questions help explain why the debate is about much more than whether a laboratory can identify an X or Y chromosome.

What Does the Future Hold?

Reproductive medicine continues to develop, and our ability to understand the genetics of embryos and reproductive cells is considerably greater than it was a generation ago.

That does not mean an era in which parents routinely choose whether to have a boy or girl is inevitable.

Scientific capability is only one part of the picture. Safety, effectiveness, medical need, ethics, regulation and the law all determine whether a reproductive technology becomes part of normal fertility treatment.

For prospective parents in the UK today, the position is much simpler than some online claims suggest. There is no reliable natural technique for choosing whether you conceive a boy or girl, and sex selection through licensed fertility treatment is not permitted simply because parents prefer one sex over another.

If you are trying for a baby, identifying your fertile window can still be extremely useful. The purpose is to maximise the opportunity for conception, rather than to determine the sex of the child.

And perhaps that is the most useful distinction to remember: modern fertility science can help us understand when conception is most likely, but that should not be confused with a proven natural method of deciding who that future baby will be.

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