Experts believe that too much alcohol interrupts the female natural cycle and may also damage the egg.
But they say women can still drink small amounts when trying for a baby as there is no evidence the odd glass hampers fertility.
Danish researchers studied 6,120 women from aged 21 to 45 who were trying to conceive, from June 2007 to January 2016.
They all completed questionnaires on how often they had alcohol and exactly what they drank.
The results published in the BMJ show that women who had at least a 250ml glass a day – equivalent to 14 servings a week - were 18 per cent less likely to conceive over a year.