How much alcohol is safe for a pregnant woman to drink?

For a question that affects so many people, surprisingly little research has been done, health experts who reviewed the scant evidence said Tuesday.

While there is widespread awareness of foetal alcohol syndrome, which can cause brain damage in unborn babies whose mothers drink, nobody knows how much it takes, or whether there is a safe limit for pregnant women to enjoy an occasional tipple.

A trawl for research on the topic found "a surprisingly limited number" of studies into low alcohol consumption during pregnancy, a team wrote in the journal BMJ Open.

And given the "paucity of evidence", the advice for now must remain "better safe than sorry," the researchers concluded.

The team searched far and wide for data on pregnant women who had imbibed four units per week -- a total of 32 grammes (1.1 ounces) or 40 millilitres of pure alcohol -- considered in Britain as "light" consumption. 

A unit in Britain is about half a pint of beer, half a glass of wine, or half a shot of the hard stuff. 

The recommended British limit for adults is 14 units, but for pregnant women, the advice is complete abstinence.

Guidelines differ between countries, but the issue is controversial.

According to the authors, up to 80 percent of mothers-to-be in Britain, Ireland, New Zealand and Australia drink some alcohol while pregnant.

A study earlier this year in 11 European countries said that about 16 percent of expectant mothers overall reported drinking some alcohol, ranging from 29 percent in Britain, 27 percent in Russia and 21 percent in Switzerland, to just over four percent in Norway.

- Less judgment -Earlier this year, British Pregnancy Advisory Service, which offers assistance to pregnant women, urged officials not to "overstate the risks from consuming small amounts of alcohol during pregnancy".

In France, winemakers took issue with the government over plans to enlarge a pregnancy alcohol warning on wine bottles, and activists took to social media to accuse the authorities of "terrorising" pregnant women.

The latest paper, based on a review of 26 studies with relevant data, does not resolve the lack of clarity.

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.

"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.

"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.

However, women who have had a drink while pregnant, perhaps unwittingly, "should be reassured that they are unlikely to have caused their baby considerable harm," the team wrote.

Experts not involved in the study welcomed its contribution to the limited knowledge pool.

While it does not say light drinking is safe, the research does highlight the weak evidence on which government advice is based, they said.

James Nicholls, research director at the charity Alcohol Research UK, said the findings "should caution us not to create a situation where mothers-to-be are made more anxious, or subject to unnecessary moral judgment, on the issue of very light alcohol consumption."

Added David Spiegelhalter of the University of Cambridge, "with luck this should dispel any guilt and anxiety felt by women who have an occasional glass of wine while they are pregnant."