Women who drink a large glass of wine each night may harm their chance of having a baby, a study suggests.
Researchers found that the chance of conceiving within 12 months drops by 18 per cent for those who regularly drink around 240ml of wine a night, the equivalent of a large glass, two bottles or beer, or two shots of spirits.
Danish researchers looked at more than 6000 women aged between 21 and 45 who had tried to get pregnant for a year and matched the result against drinking habits.
In women who drank 14 or more servings of alcohol a week - the equivalent of seven large glasses of wine - the chance of conceiving dropped by 18 per cent.
It means that one in five women who would otherwise have got pregnant failed to conceive because they drank too much.
Lead author Ellen Mikkelsen, of Aarhus University Hospital in Denmark, said: "For many women of reproductive age, alcohol consumption is an integral part of their lifestyle. In this prospective study of women trying to conceive, consumption of the highest amount of alcohol was associated with a decrease in fecundability [probability of being able to conceive] compared with no alcohol consumption."
The level at which it begins to impact upon fertility is around 16 units a week, where a unit is the equivalent of around 87ml of wine.
However, there was no impact for women who drank less.
Experts in Britain said the study showed that drinking occasionally was unlikely to impact on pregnancy.
Annie Britton, from University College London, said the results offer reassurances to couples trying to get pregnant and suggests that "total abstinence may not be necessary to maximise conception rates" because "if alcohol is consumed moderately, it seems that this may not affect fertility".
"It would be wise to avoid binge drinking, both for the potential disruption to menstrual cycles and also for the potential harm to a baby during early pregnancy," she added.
"If a couple are experiencing difficulty in conceiving, it makes sense for both partners to cut down on their alcohol intake."
The research was published in the BMJ.