"We were surprised that this very important topic was not researched as widely as expected", study co-author Loubaba Mamluk of the University of Bristol's School of Social and Community Medicine told AFP.
The British researchers agree that women should not drink while pregnant as a precaution, but said women who had a drink while pregnant are unlikely to cause their baby any lasting harm.
But the Bristol team emphasised that their review, which was funded by the National Institute for Health Research, referred to light drinking and that the effects they found were small.
Women are officially advised to avoid all alcohol while pregnant - but that recommendation is based less on hard scientific data and more on the principle of "better to be safe than sorry".
The team of researchers used all available research that has been carried out on the subject and found no evidence of harm other than an association between light drinking (up to four units of alcohol per week) and smaller babies.
They highlighted that another reason to be cautious was that the association might not reflect a direct causal link.
In the United Kingdom, health officials recommend that men and women drink no more than 14 units a week.
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Professor Elliot says there is a paucity of evidence of harm in offspring exposed to low levels of alcohol, but just because we can't see damage doesn't mean there is no damage.
An analysis of 26 relevant studies from a pool of nearly 5,000 articles found a lack of data about women who drink alcohol occasionally as opposed to those expectant mothers who drinking moderately or excessively.
The UK's Chief Medical Officer commissioned the review into alcohol's effect on pregnancy because of how much "tension and confusion" the matter sparks between health professionals and women who are with child.
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Prof David Spiegelhalter, from the University of Cambridge, said: "A precautionary approach is still reasonable, but with luck this should dispel any guilt and anxiety felt by women who have an occasional glass of wine while they are pregnant".
"We believe women should have access to high-quality, evidence-based information on matters relating to pregnancy, are capable of making the choices that are right for them, and should be trusted to do so".
"While this study adds to the evidence that drinking one to two units of alcohol a week after the first 12 weeks of pregnancy is unlikely to have a harmful impact on the baby or pregnancy, we can not rule out the risks altogether".
"In the absence of strong evidence, advice to women to steer clear of alcohol while pregnant should be made on the basis that it is a precautionary measure and is the safest option", she said.