Downing just over two drinks a week in very early pregnancy could be enough to stunt a baby’s brain development, warn scientists.
Even if the mother stops, the alcohol raises the risk children will suffer later from psychological and behavioural disorders, such as anxiety and depression.
Many women may not know they are expecting in the first six to seven weeks of pregnancy but having 16 drinks in this time is enough to cause damage, said the study.
Around 40 per cent of women in the UK drink while expecting, one of the highest rates in Europe.
Lead author, doctoral student Briana Lees, said: ‘Our research found even small amounts of alcohol while pregnant can have a significant impact on brain development.
‘Previous research has shown that very heavy alcohol use during pregnancy can cause harm to the baby.
‘However, this study shows that any alcohol during pregnancy is associated with subtle yet significant behavioural and psychological effects in children including anxiety, depression and poor attention.’
Low-level intake was regarded as one or two drinks per occasion with a maximum of six drinks a week.
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But Miss Lees, of the University of Sydney, added: ‘Generally, the more a child was exposed to alcohol in utero, the more severe the outcomes.
‘Children experienced negative effects even if they were only exposed to low levels of alcohol during very early pregnancy and then the mother stopped drinking. The difficulty is many women don’t know they are pregnant at that early stage.’
Senior author Professor Maree Teesson said: ‘The safest option during pregnancy is to abstain from any alcohol.’
For the study, interviews were held with the parents of around 10,000 children aged nine to ten, revealing that around 25 per cent had been exposed to alcohol in the womb.
Most of this group, 60 per cent, were only exposed to low levels during the first six to seven weeks before women knew they were pregnant.
But those exposed to low levels of drinking in the womb at any time were more likely to have psychological or behavioural problems.
Differences in their brain volume and surface area were seen in MRI scan data, said the study in the American Journal of Psychiatry.
Exposure to slightly heavier drinking was found to raise the risk of having an attention deficit hyperactivity disorder by about 25 per cent.
Children whose mothers had seven or more drinks a week while expecting had a 30 per cent higher risk of oppositional defiant disorder and were generally more likely to show aggressive behaviour.
The number of drinks women had while pregnant ranged from zero to 90, with 27 the average.