Couples wanting to conceive a boy should not smoke, scientists say.
New research reported by Japanese and Danish scientists in The Lancet medical journal showed that couples who smoked at the time their child was conceived were less likely to have a son.
"If the father smokes more than 20 cigarettes a
day and the mother does not smoke, the sex ratio is significantly decreased with fewer boys than girls," Professor Anne Grete Byskov, of University Hospital of Copenhagen, said.
If both parents smoked, the chances of producing a male child were lower.
Byskov and Dr Misao Fukuda of the Fukuda Ladies Clinic in Hyogo, Japan, who conducted the study, do not know why smokers produce fewer boys.
But they suspect that sperm carrying the male Y chromosome is more susceptible than sperm with the female X chromosome to the effects of tobacco.
"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," Byskov explained.
Observational studies from Denmark, Britain, the United States, Canada and other countries have shown a decline in the male-to-female ratio of children in past decades.
Exposure to toxins such as dioxin, which can affect the male reproductive system, and stress have been suggested as possible causes.
The scientists studied the smoking patterns of the parents of 11,800 Japanese infants around the time of conception.
After dividing the couples into three groups - non-smokers, less than 20 cigarettes a day smokers and 20 plus a day smokers - they found that the male-to-female sex ratio declined with the increased number of cigarettes smoked by the parents.
It was the highest in the non-smoking group and the lowest among the top smokers.
"We suggest that periconceptual [around the time of conception] smoking of the parents reduces the frequency of conceiving male children," the scientists said in the study.
A study by Italian scientists of men who had been exposed to dioxin after an explosion at a chemical plant in 1976 found they were more likely to father daughters than sons.
- REUTERS
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