Are David and Victoria Beckham likely to have a fourth boy?

image captionHaving three children of one sex is a mere statistical quirk

Victoria and David Beckham are expecting their fourth child, but does having three children of one sex mean your next one is likely to be of the same sex?

The Beckhams already have three boys - Brooklyn, 11, Romeo, eight, and Cruz, five. It's a situation where many families hope for a girl, or a boy if they have had only girls.

And there are plenty of unscientific anecdotes from families who think they are only capable of having boys, or only girls. But what are the real odds?

"The odds are just about 50-50. It's like tossing a coin," says Peter Bowen-Simpkins, medical director at the London Women's Clinic, and a spokesman for the Royal College of Obstetricians and Gynaecologists.

"There is a slight preponderance of males."

But you have a birth rate that it is close to 50/50.

The sex of the baby is determined entirely by sperm. There are sperm that produce a boy (with a Y-chromosome) and those that produce a girl (with an X-chromosome).

And the 50/50 odds are the same for any family, even one with three boys or three girls, says Prof Richard Fleming, scientific director of the Glasgow Centre for Reproductive Medicine and a member of the British Fertility Society.

Even a family of five or six boys proves nothing.

"When you have millions of families you have a wide range of standard statistical imbalances," says Prof Fleming. "I've seen some data that suggests there are seasonal elements, but the sceptics among us take that with a pinch of salt."

There are popular books giving advice on ways to be more likely to have a baby of one sex, most notably that by the fertility expert Landrum Shettles. And you will hear stories that the father's diet, timing of sex, seasonality, and even acidity and alkalinity within the female reproductive tract, can affect the outcome.

But if there is any difference to the likelihood of boy or girl it is minimal, Dr Bowen-Simpkins suggests.

"All these things will make a difference only of 1% or 2%. If you look at three generations of a family you will find it comes out pretty well 50/50."

image captionMore boys are conceived than girls

Prof Fleming is also not convinced.

"I've heard a number of anecdotes based on the credentials of sperm, but hard evidence is negligible," says Prof Fleming.

But there are some who are at least curious about statistical imbalances within the bigger picture.

"We know of many examples where the 50:50 relationship appears to change. After wars or in times of great social stress an increase in the number of male babies born in a country is sometimes observed," explains Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield.

"We also know that men in certain occupations can have an increased chance of having a baby of one sex or another. For example, men who work as divers, test pilots, clergymen and anaesthetists are apparently more likely to have daughters."

And some people have floated the idea that "status" might affect the sex of the baby.

"It has been suggested that there may be links with an individual social status and the probability of having a boy," says Dr Pacey. "It was suggested a few years ago that statistically, American presidents have had more sons than should have occurred by chance, and that royal families across Europe, including our own, generally have more sons than daughters.

"Therefore, given the high regard that we seem to have for David Beckham in our society, it is perhaps of no surprise that he should be father to three boys and perhaps this raises the stakes for his fourth child to be a boy also."

Of course, you can select the sex of your baby. There is a process by which male-producing and female-producing sperm can be separated, and it is done in some parts of the world, says Prof Fleming.

But sex selection is illegal in the UK for social reasons.

IVF clinic consultants, like Dr Bowen-Simpkins, do get many queries about sex selections from parents of a number of children of one sex, particularly from Asian parents who have had several girls and want a boy.

But the typical circumstances where sex selection is allowed is where a serious inherited disease particularly affects one sex. One of the most common is Duchenne muscular dystrophy, which usually affects boys.

In other countries the rules are less strict and sex selection takes place, particularly in case of in-vitro fertilisation.

And whatever the scepticism about boy or girl-only families being anything more than a statistical quirk, there is nothing to say that in the future science won't establish that there is an explanation.

"If you can physically separate sperm [in a lab] you can imagine biological environments where this can happen naturally, in the woman's reproductive tract. But I've never seen any evidence that it is the case," says Prof Fleming.