Pregnant women and those trying for a baby should avoid alcohol completely, according to new government advice.
Experts say there is no proven safe limit of consumption
It replaces existing advice that one to two units such as a couple of glasses of wine per week is acceptable.
The change follows concern from some sectors that there is no safe amount of alcohol that mothers-to-be can drink.
While heavy alcohol consumption during pregnancy is known to be damaging to the unborn child, the effects of more moderate intake are less clear.
The Royal College of Obstetricians and Gynaecologists says there is no evidence that a couple of units once or twice a week will do any harm to the baby.
The Department of Health said the revision was not based on new scientific evidence but was needed to help ensure that women did not underestimate the risks to their baby.
It now says pregnant women or women trying to conceive should abstain from alcohol.
If they do choose to drink, to minimise the risk to the baby, they should not drink more than one to two units of alcohol once or twice a week and should not get drunk.
Women who are already pregnant and who have followed the earlier advice "will not have put themselves or their baby at risk", the Department reassured.
Deputy Chief Medical Officer Dr Fiona Adshead said: "We have strengthened our advice to women to help ensure that no-one underestimates the risk to the developing foetus of drinking above the recommended safe levels.
"Our advice is simple: avoid alcohol if pregnant or trying to conceive.
"This advice could also be included on alcohol packaging or labels," she added.
She said many women give up drinking alcohol completely during pregnancy.
FOODS TO AVOID IN PREGNANCY
Some cheeses, such as Camembert
Pate and liver products
Raw or partially cooked eggs
Raw or undercooked meat
Some fish, such as swordfish
Undercooked ready meals
Source: Food Standards Agency
But data suggest 9% still drink above recommended levels.
Dr Sheila Shribman, the National Clinical Director for Children, Young People and Maternity Services, said: "It is vital that we alert pregnant women and women hoping to conceive about the potential dangers of excessive alcohol consumption during pregnancy.
"Although there is still scientific uncertainty about the precise impact of excess alcohol on unborn babies we believe the time is right to introduce a strong consistent approach across the whole of the UK."
Dame Karlene Davis, general secretary of the Royal College of Midwives, said: "We do not live in an ideal world, and we know that some women may drink alcohol when pregnant, but our advice remains that they should not."
FOETAL ALCOHOL SYNDROME
Abnormal facial features
Central nervous system abnormalities
Impaired learning and memory skills
Behavioural problems such as hyperactivity
Jane Brewin, chief executive of the baby charity Tommy's, agreed, saying: "There is no proven safe level of alcohol to drink during pregnancy because any amount can pass through the placenta to the baby."
There is no consistent evidence that low to moderate consumption of alcohol during pregnancy has any adverse effects, although there is some evidence that binge drinking can affect neurodevelopment of the foetus.
And heavy consumption can cause serious harm, including a condition called Foetal Alcohol Syndrome.
The National Organisation on Foetal Alcohol Syndrome estimates more than 6,000 UK children are born with Foetal Alcohol Spectrum Disorder each year.
The drinks industry said it was in discussions with government about what more it could do to effectively communicate the new advice.
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