Mothers who smoke during pregnancy could damage the future fertility of their sons, new research has found.
Experts used mice to test the long-term effects of mothers smoking during pregnancy and breastfeeding.
They discovered that male pups exposed to cigarette toxins in the womb or early life were infertile or had problems with fertility in adulthood.
Professor Eileen McLaughlin, who led the study, said: "Our results show that male pups of 'smoking' mothers have fewer sperm, which swim poorly, are abnormally shaped and fail to bind to eggs during in vitro fertilisation studies.
"Consequently, when these pups reach adulthood they are subfertile or infertile.
"This is the first time we have been able to prove conclusively that male baby exposure to cigarette toxins in pregnancy and early life will damage later-life fertility.
"Although this is a study in mice, the findings are relevant to human health as many men, now in their 30s and 40s, were exposed to cigarette toxins in the womb when it was less well known that smoking affected babies' health.
"These men have difficulty conceiving and this is associated with production of low numbers of poor quality sperm in their semen.
"Unfortunately about 25% of young women today continue to smoke when they are pregnant and/or breastfeeding - thereby potentially damaging their sons' fertility."
To carry out the tests, researchers exposed 27 female mice to smoke equivalent to 24 cigarettes a day in humans.
Their litter was compared to that of 27 mice exposed to normal air, with the total 108 male offspring examined through adulthood.
Professor Sheena Lewis, professor of reproductive medicine at Queen's University Belfast and chairwoman of the British Andrology Society, said: "This is a well-designed and carefully executed study.
"It was performed in an animal model as such mechanistic experiments are impossible and unethical in humans.
"However, the results are very clear. Mums who smoke when pregnant are potentially doing irreversible damage to their sons' fertility.
"If these mums care about their sons' future happiness and want grandchildren, they should stop smoking during pregnancy."