Smoking is bad for boys, even before they're born, it seems.
A new study says parental smoking reduces the chance a boy will ever be born.
Stress and smoking reduce chance of boy
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The boy-girl ratio was sharply skewed in favour of girls for parents who smoked around the time of conception, says a report in this week's issue of the journal The Lancet. The more the parents smoked, the less likely they were to have a bouncing baby boy.
The report comes from what might seem to be an unusual collaboration, involving scientists in Japan and Denmark. However, the teaming makes excellent sense, says Anne Grete Byskov, a professor of reproductive biology at the University Hospital of Copenhagen and a member of the research group.
The idea originated with Misao Fukuda, a Japanese researcher who has done research indicating that any kind of stress early in pregnancy reduces the ratio of baby boys. For example, he found a significant drop in the percentage of boys born to parents who lived through the massive 1995 earthquake in the Kobe region of Japan.
The well-organised Danes have a data collection system that allowed testing of Fukuda's hypothesis. The researchers recorded the sex of some 11 000 babies of women attending clinics in Denmark. Each mother was asked about whether she and the baby's father had smoked around the time of conception, and how much they had smoked - "not at all," "less than a pack a day," "more than a pack a day."
Overall, the ratio of boys to girls was about even - 1.043 boys to every one girl. However, for the non-smoking parents, the ratio was 1.2 in favour of boys. That ratio dropped steadily with the smoking habit of either father or mother.
For example, the boy-girl ratio was 0.98 when the mother-to-be didn't smoke but the father smoked more than a pack a day. It was lowest when both parents smoked a lot - they were 20 percent more likely to have a girl rather than a boy.
Is it stress or smoking?
"We can't be sure that this is caused by cigarette smoking itself," says Byskov. "Why do people smoke cigarettes? Perhaps they are more stressed in general."
However, she has some theories to explain the gender discrepancy.
Early on, it seems stress is not good for males, she says. Chromosomes determine sex: A female has two X chromosomes, a male has an X and an Y - with the Y being significantly smaller.
"It could be that the male conceptus is more fragile and more sensitive to the hostile medium caused by smoking," she says.
The effects of stress could also come even earlier, Byskov says.
"It would be interesting to do research on sperm mobility in the unfavourable medium caused by smoking," she says. "The ability to fertilise might be affected."
Smoking a hazardous method of choosing sex of child
Dr Nancy Green, acting medical director of the March of Dimes Birth Defects Foundation in the USA, worries that some couples who want a girl baby might seize on this report as a way of getting their wish.
"We would not recommend smoking for sex selection," Green says. "It has a negative effect on health - the health of the sperm, the health of the mother, the health of the early foetus."
Smoking has been shown to increase every hazard of pregnancy, she says, including placental bleeding, premature delivery and small babies.
"There are lots of reasons why people who are contemplating pregnancy shouldn't smoke," Green says.
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