Couples who smoke around the time of conception could have a reduced chance of conceiving male offspring, suggest authors of a research letter in a recent issue of The Lancet. The male to female ratio of children has declined substantially over the past few decades. The reason for this reduction is not clear, but it has been suggested that chronic exposure to toxic environmental agents that predominantly affect males and the male reproductive system could lead to a lower male to female birth ratio.
Misao Fukuda from Hyogo, Japan, and colleagues from Japan and Denmark recorded the sex of around 11,800 infants. Each mother was questioned about her and her partner's daily consumption of cigarettes around the time of conception (from 3 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 in the group in which neither mother nor father smoked, whereas the lowest sex ratio of 0.82 was seen in the group in which both mother and father smoked more than 20 cigarettes a day. The ratio of male births was also reduced (0.98) among couples where the mother was a non-smoker, but the father smoked more than 20 cigarettes a day.
Anne Grete Byskov (one of the Danish investigators) comments: "Our working hypothesis is that the sperm cells carrying the Y-chromosome-responsible for male children-are more sensitive to unfavorable changes caused by smoking than sperm cells with an X-chromosome. Such affected Y-sperm cells might be less prone to fertilize 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."
