They said Government recommendations that women stop drinking altogether in pregnancy was based on “generally weak” evidence.
Some said women should be informed of any potential risk and left to make up their own minds.
In the new review, researchers found very little evidence that light drinking in pregnancy causes harm to babies, including birth defects, developmental delay, behavioural problems and impaired intelligence.
However, they warned that a lack of evidence is not proof it is fine to drink and they recommended pregnant women do not drink alcohol as a “precautionary” measure.
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”.
It adds: “Drinking in pregnancy can lead to long-term harm to the baby, with the more you drink, the greater the risk.”
Up until last year, women were told they could drink up to one or two units, once or twice a week.
The new research review, published in the journal BMJ Open, was undertaken by experts from the Medical Research Council’s integrative epidemiology unit at the University of Bristol, the university’s school of social and community medicine, and University Hospitals Bristol NHS Foundation Trust.
They trawled all available research on the effects of light drinking in pregnancy (up to four units of alcohol per week).
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, such as whisky, gin, rum or vodka, or half a standard (175ml) glass of wine at 11.5% strength.
The team examined 26 available studies on the issue.
They 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 at all.
There was also a potential risk linked to premature birth, although this was less clear.
Very few studies compared light to non-drinkers and there was a distinct lack of evidence on most other outcomes for the baby, including development, behaviour, cognitive impairment, greater problems in pregnancy or a more complicated delivery.
The researchers said there was insufficient data to “make robust conclusions”, adding evidence on the effects of light drinking was “sparse”.
They called for more research on light drinking in pregnancy, including possible benefits of light alcohol consumption versus abstinence.
They added: “However, describing the paucity of current research and explaining that ‘absence of evidence is not evidence of absence’, appears warranted.”
Dr Christoph Lees, clinical reader in obstetrics at Imperial College London, said: “This meta-analysis of light drinking in pregnancy points to the generally weak evidence on which Government advice is based.
“Whilst it is possible that light drinking is associated with a slightly higher risk of having a small baby, there are other possible explanations.
A Department of Health spokesman said: “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.”