There is very little evidence that light drinking in pregnancy harms unborn babies, a review has found.
The study found an association between light drinking and smaller babies at birth but found no evidence of other harm, including miscarriage, birth defects, developmental delay, behavioural problems and impaired intelligence.
Experts welcomed the news, with some saying Government recommendations that women stop drinking altogether in pregnancy are based on "generally weak" evidence.
However, the Department of Health and the Royal College of Midwives (RCM) said the safest course of action is still for women to avoid drinking in pregnancy.
Overall, the researchers backed this view, saying the lack of evidence of harm was not the same as proof that it is fine to drink.
The review, published in the journal BMJ Open, included experts from the University of Bristol and University Hospitals Bristol NHS Foundation Trust.
They trawled all available research on the effects of light drinking (up to four units of alcohol per week) in pregnancy.
One unit is 8g of alcohol - equivalent to half a pint of beer, lager or cider at 3.5% strength, a single measure (25ml) of spirits or half a standard (175ml) glass of wine at 11.5% strength.
From 26 studies, the team found that drinking up to four units a week while pregnant, on average, was associated with an 8% higher risk of having a small baby compared with drinking no alcohol, with a range across the studies from 2% to 14%.
But they said while there was an association, this did not prove a direct cause of smaller babies at birth.
The researchers said that overall there was insufficient data to "make robust conclusions", adding that evidence on the effects of light drinking was "sparse".
Official NHS guidance from the Chief Medical Officers for the UK published last year says pregnant women should not drink because "experts are still unsure exactly how much - if any - alcohol is completely safe for you to have while you're pregnant".
Up until last year, women were told they could drink up to one or two units, once or twice a week.
A Department of Health spokesman said: "It is important to remember the purpose of these guidelines - they are low-risk guidelines.
"As the evidence is uncertain, the lowest risk approach is to avoid alcohol during pregnancy.
"As a precaution, we advise pregnant women to avoid alcohol and this advice is supported by the Royal College of Midwives and Royal College of Obstetricians and Gynaecologists (RCOG)."
David Spiegelhalter, professor for the public understanding of risk at the University of Cambridge, said the new review showed that "warnings about the dangers of drinking any alcohol at all during pregnancy are not justified by evidence.
"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."
Dr Christoph Lees, clinical reader in obstetrics at Imperial College London, said: "Whilst it is possible that light drinking is associated with a slightly higher risk of having a small baby, there are other possible explanations."
Andrew Shennan, professor of obstetrics at King's College London, said: "It has been difficult to associate low levels of alcohol intake in pregnancy and harm, and this work confirms this."
A spokeswoman for the British Pregnancy Advisory Service (BPAS) said: "What seems to lie at the heart of public messages addressing alcohol in pregnancy is whether women can be trusted to understand the existing evidence, and whether they are able to recognise the difference between light and heavy drinking.
"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."