Smokers who want to start a family should give up if they hope to conceive a son. A new study shows that smoking at the time of conception reduces the chances of having a boy.
The popular image of a couple sharing a cigarette after sex goes right out the window if the sex of any resulting children matters to you. Smoking afterwards (or before) works against producing a male heir.
The effect is stronger if both partners smoke, as opposed to just the man or woman, according to the study, which is published today in The Lancet by researchers in Japan and Denmark.
The effect occurs at the time of conception only, the report states. Smoking did not impose a "selective disadvantage" on male foetuses later in pregnancy.
The scientists recorded the sex of 11,815 infants born to 5,372 women who attended clinics at the research centres in Japan and Denmark. The women were asked about smoking habits and were divided into three groups: non-smokers, partners smoking up to 20 cigarettes a day and those smoking more than 20.
The male-to-female sex ratio declined with increasing numbers of cigarettes smoked, the authors state. The ratio of males to females born to non-smokers was 1.21 to 1, but this fell away rapidly to 0.82 to 1 when both parents smoked more than 20 cigarettes a day. The ratio was also much lower - 0.98 to 1 - when just the father smoked more than 20 a day.
The researchers put this down to the apparent increased frailty of sperms carrying Y-chromosomes, those capable of producing a boy. Far from being the tough Marlboro man of legend, "boy" sperms seem easier to kill off than "girl" sperms. Boy sperms simply do not go the distance when under stress.
