Women who smoke while they are pregnant have been told their habit may affect the fertility of their unborn sons.
A study carried out by scientists from Aberdeen University reveals that cigarette smoke can lead to significant reductions in a gene that helps sperm count.
It found that levels of the "desert hedgehog" gene (DHH) were almost halved in foetuses of pregnant women who smoked more than 10 cigarettes a day.
The researchers believe that those children who have been affected may go on to develop lower sperm counts and also experience other problems with testicle development.
The DHH gene releases the DHH molecule in the testes, which lets other cells know what they need to do and helps control normal testicle growth.
Paul Fowler, senior lecturer in reproductive physiology at Aberdeen, who led the study said: "This is the first time that the gene DHH, which plays a key role in the male's normal development, has been linked to maternal smoking and fertility problems.
"Our research suggests that lowered DHH may be a reason why baby boys of women who smoked 10 or more cigarettes a day during pregnancy were at a higher risk of abnormalities and future fertility problems."