The evidence that light or occasional drinking in pregnancy was harmful was "surprisingly limited" but scientists advised expectant moms are advised to avoid alcohol "just in case".
Last year, a report from the Centers for Disease Control and Prevention said that women should avoid drinking completely if they are not using birth control and there is any chance they might be pregnant.
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".
They reviewed all the available studies done on the topic since the 1950s and found no convincing proof that a drink or two a week is harmful.
It found "some evidence" that drinking up to four units of alcohol per week may be associated with a higher risk of having a smaller baby or giving birth prematurely - but nothing conclusive.
Many decades ago, oddly enough, it was common for doctors in Ireland to advise pregnant and nursing women to sip Guinness for what they thought could have been some health benefits, experts say.
"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 study systematically reviewed all the data from a wide range of high quality observational studies on the impact of light drinking - around 32g of alcohol, which works out at three standard drinks in Irish units.
The evidence proving light drinking was risky was "surprisingly limited", they concluded.
It looked particularly at complications of pregnancy and birth characteristics, such as miscarriage, premature birth, and undersized babies, and longer term issues, such as the developmental delays, impaired intellect and behavioral difficulties typical of fetal alcohol syndrome - outcome of heavy drinking in pregnancy.
The scientific literature lacks data on low-level alcohol consumption during pregnancy, but the new paper did a pretty thorough job of trying to examine any possible sources of information, said Dr. Robyn Horsager-Boehrer, professor and chief of obstetrics and gynecology at the University of Texas Southwestern's William P. Clements Jr.
"Formulating guidance on the basis of the current evidence is challenging".
Mothers who are consumed by anxiety and guilt for having drunk the odd glass of wine when they are pregnant should be reassured by a new study showing there is very little evidence that it harms the baby, say experts.
There was also some evidence of a heightened risk of premature birth, but this was less clear.
The evidence on how much, if any, is safe to drink, or at what stages of pregnancy, is notable by its absence, they add.
"Evidence of the effects of drinking up to 32 g/week in pregnancy is sparse".
A standard 175ml glass of red, white or rose wine with a ABV of 12 percent contains 2.1 units.