The study, published in the British Medical Journal, is the first to suggest that moderate drinking can affect women's fertility. It is known that heavy drinking reduces a woman's chances of conceiving but, until now, it was thought that the occasional glass of wine or beer would do an intending mother no harm.
The finding will add to public confusion over whether drinking is good or bad for health. A wealth of research on the effects of alcohol, much of it conflicting, has left the average drinker bemused about the number of drinks safely permitted and whether they will increase or decrease the average lifespan (see panel).
In the latest investigation, Danish researchers, who studied 430 couples aged 20-35 who were trying to conceive for the first time, found that those women drinking one to five units a week were a third less likely to conceive within six months than those who drank nothing. (A unit is a glass of wine, half a pint of beer or a single measure of spirits.)
Those who drank six to ten units a week reduced their chances by almost half, and those who drank more than 10 units cut their chances by two thirds. Drinking appeared to have no effect on the fertility of the men.
Dr Tina Jenson, of the National University Hospital in Copenhagen, said: "I expected to find an effect of alcohol but not at these low doses. That is what surprised me."
Dr Jenson said that buying a bottle of champagne and going out for a meal could be an effective prelude to sexual intercourse, but all the couples in the study were trying to start a family and did not have problems with sex.
The drinkers had sexual intercourse slightly more often than the non- drinkers, with more than a third of the drinkers reporting having sex between seven and ten times each month compared with a quarter of the non-drinkers. To correct for the effect of extra sex, the study excluded couples who did not have intercourse between day 11 and day 20 of the woman's menstrual cycle, when the chances of conceiving are highest.
Dr Jenson and her colleagues concluded: "This finding needs further corroboration [by other studies] but it seems reasonable to encourage women to reduce their intake of alcohol or not to drink at all when they are trying to become pregnant."
It is not known how alcohol affects fertility, but animal studies have shown that it can provoke spontaneous abortion.
One hypothesis is that alcohol may interfere with the process of ovulation, the transport of the egg down the Fallopian tube and its implantation in the womb after fertilisation.
Dr Jenson said: "It is fantastic that the egg can be implanted in this way. It is a delicate process and it is not difficult to imagine why women's fertility might be more susceptible to the effects of alcohol than men's."
Among pregnant women, heavy drinking is linked with a higher incidence of miscarriages, still births and premature births.
Dr Jenson said a possible reason the effects of moderate drinking on conception had been missed in earlier studies was that personal estimates of the amount drunk were notoriously unreliable. "Other studies have asked women what they drank on average.
"We went back every month and asked them what they had drunk. Some told me that they had had a hen night that month and drank 20 units. So we got a more accurate picture."Reuse content