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Friday, 19 April, 2002, 09:51 GMT 10:51 UK
Smokers produce more baby girls
Couple smoking
Smoking may affect male sperm
Parents who smoke both before and around the time of conception are more likely to have female babies, it is claimed.

The male to female ratio of children has declined substantially over the past few decades and it has been suggested environmental pollution may be to blame.

It is claimed toxic environmental agents could affect males and the male reproductive system.

The latest study suggests smoking may either damage male sperm cells or jeopardise the chance of the male foetus implanting in the uterus, or both.

We have found that the male sperm cell is more susceptible to smoking

Dr Claus Yding Anderson, gynaecologist
The study, published in The Lancet, recorded the sex of around 11,800 infants in Japan and Denmark over a seven-month period.

Each mother was questioned about her and her partner's daily consumption of cigarettes around the time of conception.

This covered the three months before the last menstruation, to when the pregnancy was confirmed.

The male to female sex ratio was calculated for three groups: men and women who did not smoke, those who smoked up to 20 cigarettes a day, and those who smoked more than 20 cigarettes a day.

The male to female sex ratio declined with increasing numbers of cigarettes smoked by mothers and fathers.

The sex ratio was 1.21 boys to every girl in the group in which neither parent smoked.

Vulnerable sperm cells

However, it fell to 0.82 boys to every girl in the group where both mother and father smoked more than 20 cigarettes a day.

The ratio of male births was also reduced to 0.98 boys to every girl among couples where the mother was a non-smoker, but the father smoked more than 20 cigarettes a day.

Dr Claus Yding Anderson, from University Hospital of Copenhagen, was one of the team carrying out the research.

He said: "We have found that the male sperm cell is more susceptible to smoking.

"Or it could be that smoking affects the implantation of the male conceptus in the uterus."

Jet pilots

His colleague, Professor Anne Grete Byskov, said: "Our working hypothesis is that the sperm cells carrying the Y chromosomes responsible for male children are more sensitive to unfavourable changes caused by smoking than sperm cells with X chromosomes.

"Such affected Y sperm cells might be less prone to fertilise and/or produce less viable embryos.

"Smoking may cause a stress effect on the sperm cell itself since the sex ratio also declined when the mother smoked but not the father."

Other studies have found jet pilots from the armed forces have a tendency to produce more girl babies.

This is also the case for divers who spend more than 20 hours diving a week.

However, globally, slightly more boys are born than girls, although girls have better survival rates.

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