The new study adds another reason to stop smoking during pregnancy, as it found the male offspring of smoking mothers could have diminished fertility.
The researchers - who were led by Prof. Eileen McLaughlin of the University of Newcastle in Australia - publish their results in the journal Human Reproduction.
They note that until now, the effects of smoking during pregnancy on male offspring fertility - as well as the mechanisms behind this - have been unclear, due to a lack of animal studies with carefully controlled environments and exposure to toxic chemicals.
With this latest study, however, the team has created the first comprehensive animal model to show how smoking can affect male offsprings' fertility.
Prof. McLaughlin explains that they used a mouse model that mimics human smoking for their study because - naturally - it would be "unethical to deliberately expose pregnant mothers and their offspring to the toxins in cigarettes."
However, she and her team say their findings are relevant to human health.
Male pups of smoking mothers 'subfertile or infertile'
To conduct their study, the researchers created a machine that pulls smoke into a nose piece and placed it on the noses of 27 female mice who then inhaled the smoke into their lungs.
In total, the mice inhaled smoke equivalent to a pack of 24 cigarettes a day in humans. Meanwhile, another group of 27 mice served as controls and were only exposed to normal air. Then, after 6 weeks, all of the mice were placed with males to reproduce. The "smoking" mice were exposed to cigarette smoke throughout pregnancy and lactation, until their pups were weaned.
The team examined all 108 male mice offspring regularly from birth through adulthood, inspecting damage to the DNA of cells involved in producing sperm, sperm counts, sperm shape and how well they swam, and the ability of the male offspring to reproduce.
The research revealed that the smoking mothers produced male pups with fewer sperm that swim poorly, are abnormally shaped and do not bind to eggs during in vitro fertilization.
Commenting on their findings, Prof. McLaughlin says:
"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."
She further explains that it is already known cigarette toxin exposure affects the stem cell population in the testes, reducing the amount of sperm produced. Additionally, oxidative stress from cigarette toxins damages cells in the testes, resulting in sperm with "abnormal heads and tails, that are unable to swim properly or successfully bind and fuse with eggs."
Despite limitations, study has human implications
The researchers admit that because their study only investigated mice using a smoking model that merely simulates human cigarette smoke exposure, this is a limitation.
However, they add that their research strengthens current findings suggesting that exposure to smoke during pregnancy impairs the fertility of male offspring - a topic they say is currently debated due to conflicting studies.
What is more, Prof. McLaughlin adds that the findings are relevant to human men, as many who are now in their 30s or 40s were exposed to cigarette toxins in utero back when the negative health consequences of smoking during pregnancy were less well known.
"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 breast feeding - thereby potentially damaging their sons' fertility."
She and her team say smoking cessation programs should carry on advocating that women avoid smoking during pregnancy and while breast feeding.
They are currently focusing on the effects of cigarette smoke exposure in the sons, grandsons and great-grandsons of mouse mothers who smoked during pregnancy, as well as focusing on the health implications for the eggs of female offspring.
Medical News Today recently reported on a study that suggested babies whose father smoked before conception have an increased asthma risk.