Men undergoing fertility treatment who have just one alcoholic drink a day reduce their chances of success by almost a tenth, a new study suggests.
Women who drink the same amount also find their chances reduced, but by a smaller percentage.
Scientists said alcohol affects fertility in men by reducing the number of sperm and altering their size, shape and motility.
It can also inhibit proper implantation in the womb and therefore increase the chance of early pregnancy loss.
A study of 27,000 adults who underwent fertility treatment, carried out by the Tongji Hospital in Shanghai, China, found that compared with people who drank no alcohol, live births fell by nine per cent in men who drank seven alcohol beverages or 84g of alcohol a week.
In women who drank 84g of alcohol, chances of pregnancy fell by seven per cent.
Dr Yufeng Li, an author of the study commented: “Couples should be aware some modifiable lifestyle factors such as drinking habits may affect their fertility treatment outcomes.
“But how these factors impact the reproductive system still needs more research to elucidate.”
The study adds to existing evidence which suggests that couples are struggling to conceive because of lifestyle choices.
One 2017 review of 185 studies, by the Hebrew University of Jerusalem, found that sperm concentrations in men decreased by up to 59 per cent from 1973 to 2011.
Some fertility experts have now advised those trying to conceive to “play it safe” and cut out alcohol altogether.
The new study, published in the journal Acta Obstetricia et Gynecologica Scandinavica, suggests abstention is best for both partners.
Dr Li said the toxicity of alcohol has been well established and the safest level of drinking is zero.
“In the process of alcohol metabolism, reactive oxygen species (ROS) may form,” Li explained.
He said excessive production of ROS increases oxidative stress, which is thought to be a contributor to endometriosis, polycystic ovary syndrome, unexplained infertility, spontaneous abortion and recurrent pregnancy loss.
“Modifiable lifestyle factors such as smoking and habitual alcohol drinking may contribute to production and exposure of ROS, which may partially explain why alcohol intake is associated with impaired IVF outcomes,” Li said.
Contrary to other studies, the team did not identify any connection between drinking coffee and fertility.
“Alcohol consumption is negatively associated with pregnancy rate of IVF treatment when women drink more than seven drinks per week,” Li said.
He added: “There was no association between caffeine consumption and pregnancy or live birth rate.”