Page last updated at 11:01 GMT, Wednesday, 23 April 2008 12:01 UK

Can you influence a baby's sex?

By Nick Triggle
Health reporter, BBC News

There are a variety of theories about influencing the sex of a baby.

But do they have any scientific merit or are they just old wives' tales?

Sperm cells
Male-producing sperm are said to swim more quickly

Recent polls are clear. Many parents, given the chance, would love to influence the sex of their baby.

Four out of 10 say they would like to choose whether they have a boy or girl, with one in five even claiming they would pay 1,000 for the opportunity.

But why are parents so obsessed about it?

Annette Briley, a consultant midwife at Tommy's baby charity, believes it is linked to the ever-shrinking size of the family unit.

She points out whereas it was not uncommon for the post-war generation to have four or five children, most parents now opt for just two.

Control

"When our grandparents were having lots of children, there was always a good chance that they would have a mix of boys and girls.

"But now we have fewer that is not so likely.

"I guess it is also related to the nature of our society where we have so much control and I think that extends to choosing the make-up of our family."

And for couples wanting to influence their child's sex, there is no shortage of advice.

Fertility doctors already have the know-how to be able to decide the sex of a baby through embryo selection, but that is banned in the UK except for medical reasons.

There is very little scientific evidence that any of these lifestyle factors really have an impact
Dr Simon Fishel, fertility expert

As a result, parents are left to sift through a selection of what some dub old wives' tales.

Many of them relate to the environment of the vagina at the time of sex.

The theory goes that male sperm struggle more than female sperm in the naturally acidic conditions.

In the 1980s, douching, flooding the vagina with an alkaline solution such as baking soda before sex, was used.

This environment theory has also been used to lend support to the suggestion sexual positions can influence the reproduction process.

Since the vagina is known to be less acidic closer to the cervix, it has been claimed that having sex with the woman on top or the man behind, during which penetration is deeper, is more likely to lead to a boy.

Another theory is that as X-chromosome carrying sperm, which produce females, live longer, but swim slower than their Y-chromosome - male-producing - counterparts, it may increase the odds of a girl by having sex several days before ovulation and then abstaining so that the male sperm die.

This certainly seems to have gained some credence in the 21st century if the booming sales of ovulation sticks are anything to go by.

And then there is diet. Previous research has suggested potassium, found in the likes of meat and bananas, helps Y-chromosome sperm thrive, while the magnesium in nuts, soya beans and green leafy vegetables is good for X-chromosome sperm.

The latest study takes this a step further, suggesting a high calorie diet increases the odds of a boy.

But despite some parents swearing by these techniques, experts remain unconvinced.

Odds

Dr Simon Fishel, a leading fertility doctor and head of the CAREfertility group of clinics, said: "There is very little scientific evidence that any of these lifestyle factors really have an impact. At most they will only alter the odds slightly.

"But that is not to say people don't believe them. I get one or two emails each month with parents asking me about how they can have a boy or a girl.

"In the end I don't think we should decry this too much as long as every child is loved and cherished."

And no matter how silly some of these techniques seem, spare a thought for the French in the 18th century.

It was widely believed that if a man tied off his left testicle, a boy would be more likely.

The theory was based on the mistaken belief that sperm from each testicle were sex-specific.


SEE ALSO
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27 Mar 07 |  Health

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