Parents who smoke produce fewer boys, according to research on Japanese couples. The researchers don?t know why this happens, but say both the mother's and father's smoking habits are important.
Stress, temperature, birth order and even the number of wives in a man's harem are all known to influence the relative proportions of girls and boys at birth. But this is the first time the sex ratio of babies and smoking have been linked.
The researchers questioned over 5000 Japanese women on their smoking habits and those of their partner around the time they conceived their children. The 11,815 births reported were then split into groups based on parental smoking habits and the team compared the ratio of boys to girls in each category.
If neither parent smoked, boys made up about 55 per cent of births. But if both mom and dad smoked over 20 cigarettes a day, this dropped to 45 per cent.. Even if only the father smoked and was a light smoker, the proportion of boys was still lower.
"We cannot say where the effect is," says Anne Byskov at the University Hospital of Copenhagen. She speculates that Y bearing sperm or male fetuses or both are more sensitive to the effects of cigarette smoke.
Alexander Lerchl of the International University Bremen, Germany says, "Y bearing sperm may be a bit faster swimming, but not as long lasting. "
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People living in southern Europe have more male children than those in the north, but for some reason the reverse is true in the U.S. Victor Grech and his team at St. Luke's Hospital in Malta used data from the World Health Organization to compare the male to female birth ratio in Europe and North America over 50 years.
They split Europe into three separate categories: southern Europe, central Europe and Nordic countries, and North America into Mexico, the U.S. and Canada. They found that in Europe, significantly more boys were born in southern Europe, such as Greece and Spain. "And this fits with an earlier German study that plotted male and female births, and found more males when the conception date was in the hottest months," Grech says. "But then we looked at North America, and the ratio was inverted, with significantly more males born in Canada."
Between 1950 and 1999, the team found a shortfall of about 200,000 boys in central Europe, compared with 12,744 in southern Europe. In Mexico, the shortfall for the period between 1958 and 1997 was 521,789, compared with 21,993 in Canada. Research shows the average ratio of male to female births over the past 50 years has been decreasing worldwide. Some scientists think male embryos may be more susceptible to environmental toxins.
"I honestly haven't got a clue about what is behind our findings," Grech says. "But it is possible that exposure to environmental toxins such as dioxins could be involved."The number of baby boys born to people living in the Italian town of Seveso declined dramatically after an explosion at a herbicide plant in 1976 released high quantities of dioxins into the air. Grech says, "We now need to breakdown the data and look at trends over time - that might indicate whether exposure to environmental toxins accounts for the results."
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