Most people know where babies come from and what to do if you want one. Historically, whether you get a baby boy or a baby girl has been a random process. For centuries couples have tried prayer, black magic or even testicle cuffs to try and select the sex of their offspring, only succeeding in a predictable 50% of cases. Now, at the start of the 21st Century, there are techniques available to do this claiming a success rate of over 90%.

The sex of a child is determined by the sex chromosome carried by a man's sperm. An X chromosome produces a girl, a Y chromosome gives a boy. Sex selection techniques work either by separating the sperm carrying each type of chromosome before fertilisation occurs; or by selecting embryos fertilised by the desired carrier, before implantation in the potential mother's womb.

So if you have always wanted a boy but have so far had a series of girls; or if you long for a little girl and know you'd be disappointed to give birth to a son then read on. But don't get too hopeful yet, sex selection is expensive, only available in some countries, and then only in certain cases.

Preimplantation Genetic Diagnosis

Preimplantation genetic diagnosis (PGD) is a potentially very powerful technique made available by IVF treatment and the development of genetic sequencing tools. IVF (In Vitro Fertilisation) treatment was developed to treat infertility. It involves a woman taking hormone drugs to help her produce more eggs. These are collected and mixed with a sperm sample, and fertilisation occurs outside the body. One or two resulting embryos are then placed inside her womb, and with luck a pregnancy will follow.

However, allowing fertilisation to occur outside of the body allows further techniques which could do more than just help overcome infertility. In PGD, a few days after fertilisation, a cell is taken from an embryo and the DNA it contains analysed. This allows a male or female embryo to be selected or rejected*.

IVF treatment is widely available. However the use of PGD for non-medical purposes is currently illegal in the UK and many European countries. It is available in the USA and elsewhere.

Sperm Sorting

Sperm sorting allows sex selection without IVF by separating the X and Y carrying sperm before they are used to impregnate a woman. Separation is possible because the X chromosome contains more DNA than the Y chromosome. It has recently been developed for use with human sperm by the Genetics and IVF Institute in Virginia.

The method attaches fluorescent probes to the sperm DNA. When illuminated by a laser, the larger X chromosomes will fluoresce brighter than the Y chromosomes. A flow cytometer can then separate the two leaving an X or Y enriched sample. The technique (given the trade name MicroSort) is claimed to have a 91% success rate to select a girl, and 73% for a boy.

MicroSort is currently only available in the USA, and is currently only offered to couples who either wish to avoid passing an X-linked genetic disorder to their children*; or who already have one or more children of one sex, and wish their next to be of the opposite gender. They charge $2500 per try, which is not refunded if you do not become pregnant or conceive a child of the other gender.

Tipping the Balance

If you don't are $2500 spare; can't get a US visa; or simply think the last two options are too much hassle, it may be possible to improve your chances of conceiving a child of the desired sex using a simpler method.

Your chances of conceiving a child of the desired sex are pretty even, worldwide 106 boys are born for every 100 girls. However it appears that various environmental factors can shift the ratio. For instance, it has been observed in hot weather more boys are conceived (with more girls during a cold spell). Other studies have shown vegetarian women, and parents who smoke are slightly more likely to have a daughter than a son. Although some have dismissed these results as a statistic fluctuation, there is a growing amount of evidence that it is possible to shift the balance in your favour.

One theory behind some of these variations is that sperm carrying an X chromosome usually survive longer than those carrying a Y chromosome, however Y sperm move faster. When the going gets tough, the X carrying cells are more likely to survive. However, if conditions are good, Y carriers are more likely to reach the woman's egg first.

One technique to take advantage of this theory is to carefully time when you have sex. If you want a boy, have sex the same day you ovulate so the faster moving Y sperm will be most likely to fertilise the egg. If you want a girl, have sex two to three days before ovulation. Then by the time an egg is released, most surviving sperm will be X carriers.

This technique is widely publicised, but there is little - if any - reliable information on its success rate. Some people claim it does not work, others say it can if you get the timing right.

Boys or Girls

A big question concerning sex selection is whether more people want boys or girls. And if the treatment was readily available to all, what would happen to the sex ratio of our society?

In many parts of Asia there is a clear cultural preference for male children. This, combined with the 'one child' policy, means that in China sex selection is already practised in the form of selective abortions and infanticide. The male/female births ratio is around 120:100, and even higher in some rural regions.

In Europe and America, however, surveys show most people want a 'balanced' family, with at least one boy and girl. However one American study showed that 81% of women and 94% of men wanted their first child to be a boy.

Despite this male bias, statistics from the GIVF Institute show most of their clients want girls. Some want a female child to avoid passing an X-linked disorder to a son; however most already have one or more boys, and want a little girl.

Reading the postings on parenthood forums it is clear that a lot of women (and at least a few men) have always longed for a daughter. There are stories of pregnant women who buy lots of baby clothes with a clear picture of the girl who will wear them, only to give birth to a baby boy.

Ethical Questions

Nearly all developments in human genetics raise ethical questions about how they should and should not be used. Sex selection is no exception to this. So before you book an appointment at the nearest PGD clinic, it is sensible to ask yourself 'is it really right for people to select the sex of a child?'.

One argument is that sex selection other than for medical reasons is trivial and treats children as commodities. You should accept a child as a 'gift', and not select the sort of child you want. If you wish to use sex selection you may have a strong image of the child you want. What if the sweet little girl you always wanted shuns the pink things you buy, cuts her hair short and wants to play football?

However many people who do not 'select' a baby still pressurise their child to fit the image they have of them. Choosing the sex of your child is certainly selfish, but then most people have children for selfish reasons anyway. If only to brighten their lives or satisfy their hormone-driven instincts.

Some pro-life groups have expressed concern about rejecting embryos of the undesired sex. To what extent an embryo of a few cells is an 'unborn child' or has any spiritual significance is a personal decision.

This technology is currently very expensive, and only available to a limited number of people. However, it is quite feasible that in the future it could be readily available to people everywhere. This could have a big impact on society.

In the Far East, it could help reduce the number of abortions and infanticide cases. However, it would raise the male/female ratio. A large gender imbalance could lead to problems, as a large number of men compete for partners. For this reason it is unlikely authorities would allow sex selection on a large scale.

In the West if the surveys are reliable, we would see an increase in mixed families. This may not be a problem, but the preference for firstborn boys has concerned some feminists, who see it as pushing men into a leading role and relegating women to little sisters. Eldest children are often more successful and better educated than their younger siblings.

Also, there are some feminists who want to use sex selection. They want girls.

The Human Fertilisation and Embryo Authority which regulates IVF and the storage of frozen embryos in the UK have carried out a consultation on sex selection using members of the public.