Whats more, just two weekly units may make a difference.
The Department of Health recommends that pregnant women, or those trying to conceive, should not drink alcohol at all. If they chose to drink, the governments advice is to not have more than one to two units of alcohol once or twice a week.
Women who drank more than the recommended two weekly units were twice as likely to give birth to an unexpectedly small or premature baby than women who abstained completely.
But the study found that even women who didnt exceed the maximum recommended alcohol intake during this period were still at increased risk of a premature birth, even after taking account of other influential factors.
Drinking during the period leading up to conception was also linked to a higher risk of restricted foetal growth, indicating that this may also be a critical period.
Professor Janet Cade from the University of Leeds School of Food Science and Nutrition
said: Since pregnancy is such a special time for women it would be sensible for anyone who is thinking about getting pregnant to avoid alcohol during that time.
The researchers based their findings on responses to food frequency questionnaires by 1264 women at low risk of birth complications in Leeds. All the women were part of the Caffeine and Reproductive Health (CARE) study, looking into links between diet and birth outcomes.
Some 13% of the babies born were underweight, and 4.4% were smaller than would be expected. A similar proportion (4.3%) were born prematurely. Drinking during the first three months of pregnancy was most strongly linked to these outcomes.
The mums-to-be were asked to identify how often, and what type of alcohol they drank, at four key time points - in the four weeks before conception and in each of the subsequent three months (trimesters) throughout the pregnancy.
Alcohol consumption was significantly higher before conception and in the first three months of pregnancy than in the subsequent two trimesters. Consumption rates averaged 11 units in the first trimester, four in the second and just under two units a week in the final trimester.
Over half (53%) of the women drank more than the recommended maximum two weekly units during the first trimester. And almost four out of 10 said they drank more than 10 units a week in the period leading up to conception.
The research also found that those who drank more than two units a week were more likely to be older, educated to degree level, of white ethnicity, and more likely to live in affluent areas.
The research was published online in the Journal of Epidemiology and Community Health.
For further information
Authors Camilla Nykjaer and Professor Janet Cade are available for interview. Contact the University press office on 0113 343 4031 or email firstname.lastname@example.org