There is no safe amount of alcohol that mothers-to-be can drink, experts believe.
Abstinence is best, say experts
Even the small amount advised by the government can harm the unborn child, a UK conference on foetal alcohol syndrome heard this week.
Dr Raja Mukherjee of St George's Medical School believes many more babies - up to one in every 100 - are affected than currently recognised.
Many are incorrectly labelled with 'behavioural' problems, he said.
Heavy alcohol consumption during pregnancy is known to be damaging to the unborn child, which is why the government sets a limit of one to two units of alcohol per week for mothers-to-be.
But Dr Mukherjee told delegates at an Foetal Alcohol Syndrome Aware UK conference in Wigan that
studies show any amount of alcohol can be damaging.
He said one in every 100 babies born is damaged by their mother drinking while pregnant, but many cases are going unrecognised.
No safe level
According to Dr Mukherjee, obvious cases of foetal alcohol syndrome - a group of problems in children born to mothers who drank alcohol during their pregnancy, which includes abnormal facial features and nervous system problems - are recognised and detected.
But many children who develop behavioural problems as a result of exposure to alcohol in the womb are incorrectly diagnosed as having conditions such as attention deficit hyperactivity disorder.
The real culprit, alcohol consumption, goes unnoticed in these milder forms, collectively called Foetal Alcohol Spectrum Disorder, he said.
He blamed binge drinking for the high rates of damage.
"The brain develops throughout pregnancy and the most vulnerable period is the first three months - the first trimester - when people may not even know they are pregnant."
He said the government should warn women that any amount of alcohol could damage their unborn child.
"The current recommendation of one to two units is ambiguous.
"The only guaranteed safe way is not to drink at all when you are pregnant," he said.
A spokeswoman from the National Organisation on Foetal Alcohol Syndrome said: "We do not want to panic any woman who is pregnant and may have had a couple of drinks. The chances are their child will be fine.
"But there is a risk if you drink alcohol during pregnancy. The only way you can be certain is to abstain from alcohol," she said.
A spokeswoman from the Department of Health said:
"We would be interested to see any further research in to this area but current evidence does not justify changing our advice."
In May, Lord Mitchell from the National Organisation on Foetal Alcohol Syndrome highlighted the issue to the House of Lords.
He said the government should be doing more.
"The government is being really complacent about it. It's sort of saying 'have a few drinks and that's OK', but our feeling, and certainly the evidence that we have seen in other countries, is that no drinking is the best policy."
He said alcoholic beverages should carry warning labels about the potential for damage to unborn children in the same way that they already do in France and the US.
Later this week, experts will debate the problem at House of Commons.