A 15-ounce can of beer which is 4.5 percent alcohol is 2 units of alcohol, while a single shot of spirits is 1 unit of alcohol, explained the website www.nhs.uk/Livewell/alcohol/Pages/alcohol-units.aspx.
Professor Janet Cade of the University of Leeds' School of Science and Food Nutrition and colleagues linked women drinking alcohol -- just two weekly units of alcohol -- during the first three months of pregnancy to a higher risk of having a premature or unexpectedly small baby.
The research also found those who drank more than two units a week were more likely to be older, were college educated, of white ethnicity and more likely to live in affluent areas.
Britain's Department of Health recommends pregnant women, or those trying to conceive, to not drink alcohol at all. But, if they chose to drink, the government's advice is to not have more than one to two units of alcohol once or twice a week, Cade said.
However, the study found even women who didn't exceed the maximum recommended alcohol intake during this period were still at increased risk of a premature birth. Drinking alcohol during the period leading up to conception was also linked to a higher risk of lower fetal growth, Cade said.
The study involved 1,200 women in Leeds, England, at low risk of birth complications. All the women were part of the Caffeine and Reproductive Health study, looking for links between diet and birth outcomes.
The study, published in the Journal of Epidemiology and Community Health, found 13 percent of the babies born were underweight, and 4.4 percent were smaller than would be expected. A similar proportion, 4.3 percent, were born prematurely. Drinking during the first three months of pregnancy was most strongly linked to these outcomes, the researchers said.
The study also found 53 percent of the women drank more than the recommended maximum two weekly units during the first trimester and almost 4-of-10 said they drank more than 10 units a week in the period leading up to conception.