Smoking couples 'more likely to conceive a girl'

Couples who smoke at around the time of their child's conception significantly increase the chances of having a girl, scientists have found.

Couples who smoke at around the time of their child's conception significantly increase the chances of having a girl, scientists have found.

The discovery may be explained by the fact that sperm cells which carry the male-bearing Y chromosome are more susceptible to the toxins in cigarette smoke and thus may be less prone to fertilise.

A team from Japan and Denmark uncovered the link after investigating the smoking habits of the parents of 11,815 newborns.

Each of the 5,372 mothers in the study was asked about her and her partner's daily consumption of cigarettes from three months before the last menstruation to when the pregnancy was confirmed.

The scientists calculated the male-to-female ratio of babies for three groups: men and women who did not smoke at all; those who smoked up to 20 cigarettes a day; and those who smoked more than 20 cigarettes a day.

They found that the proportion of boys to girls was highest in non-smokers, with 1,975 boys and 1,627 girls, and lowest in the group where both mother and father smoked, with 255 boys and 310 girls.

There were also fewer male babies when either the father alone or the mother alone smoked. There was a clear increase in the probability of a girl baby when there was increased cigarette consumption, indicating that it was cigarettes themselves which appeared to reduce the chances of a boy.

Professor Anne Grete Byskov of the University Hospital of Copenhagen, who led the investigation said cigarette smoking might help to explain the trend seen in recent decades of there being fewer newborn males in many of the developed countries, including England, Wales, the United States and Canada.

"Our working hypothesis is that the sperm cells carrying the Y-chromosome responsible for male children are more sensitive to unfavourable changes caused by smoking than sperm cells with an X-chromosome," Professor Byskov said.

The results of her investigation are published in The Lancet today.

"Such affected Y-sperm cells might be less prone to fertilise 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," she said.

Statistically the chances of a boy or girl are 50:50. There is normally a slightly higher probability of a male conception, although this is offset by there being a higher risk of spontaneous abortions or prenatal death in boys.