Drinking even small amounts of alcohol during the first three months of pregnancy may increase a woman's risk of giving birth prematurely or having an unexpectedly small baby, a new study has found.

UK researchers looked at over 1,200 pregnant women who were at low risk of experiencing birth complications. All were questioned about their alcohol intake at four different points - in the four weeks before conception and during each subsequent trimester.

The study found that alcohol consumption was much higher during the weeks before conception and in the early part of the pregnancy, compared to the second and third trimesters.

During the first trimester, at least half of the women drank more than two units of alcohol per week, while almost four in 10 said they consumed at least 10 units per week in the period just before conception.

One unit of alcohol is equivalent to one small (100ml) glass of wine, one glass of beer or one pub measure of spirits, such as vodka or whiskey.

The researchers noted that those who consumed more than two units per week during the first trimester were more likely to be older, educated to degree level, from more affluent areas and of white ethnicity.

The study found that 13% of the babies born to women taking part in this study were underweight and just over 4% were smaller than would be expected. Over 4% were also born premature.

The researchers from the University of Leeds found that drinking alcohol during the first trimester was most strongly linked to these outcomes.

In fact, women who consumed more than two units of alcohol per week during the first trimester were twice as likely to give birth prematurely or have an unexpectedly small baby, compared to women who consumed no alcohol during this time.

However, even women who drank less than two units of alcohol per week still had an increased risk of giving birth prematurely.

The researchers also noted that drinking in the weeks before conception was linked to an increased risk of restricted foetal growth. This appears to be a critical time, they insisted.

"Our results highlight the need for endorsing the abstinence-only message, and further illuminate how timing of exposure is important in the association of alcohol with birth outcomes, with the first trimester being the most vulnerable period," they said.

Details of these findings are published in the Journal of Epidemiology and Community Health.

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