Women who drink as little as one small glass of wine a week while pregnant could risk reducing their child's IQ, a study has suggested.
The effect is only felt by children who have genes that make them vulnerable to alcohol, scientists found.
For these children, even being exposed to relatively small amounts of alcohol before birth could have a significant impact on intelligence at age eight.
It is already well known that heavy drinking in pregnancy can severely harm an unborn baby.
But the picture is less clear with regard to light or moderate alcohol consumption.
Some guidelines urge mothers-to-be to avoid alcohol completely, while others suggest a moderate intake is safe.
The new findings, published online in the journal Public Library of Science ONE, suggest that genes have a strong bearing on the effect of alcohol in the womb.
Scientists studied four genetic variants in more than 4,000 children that influence the body's ability to metabolise, or break down, alcohol.
They also questioned mothers about their drinking habits while they were pregnant.
Test results showed a lowering of IQ for genetically susceptible children whose mothers consumed between one and six units of alcohol per week.
Read our tips for encouraging a health pregnancy
A unit of alcohol is roughly equivalent to one small glass of wine, a pint of beer, or a single measure of spirits.
At eight-years-old, IQ was reduced by almost two points on average for each of the genetic variants a child possessed - but only if his or her mother drank while pregnant.
Children whose mothers avoided alcohol completely were not affected, even if they had alcohol-sensitising genes.
Lead researcher Dr Ron Gray, from Oxford University, said: "This is a complex study but the message is simple: even moderate amounts of alcohol during pregnancy can have an effect on future child intelligence.
"So women have a good reason to choose to avoid alcohol when pregnant."
Colleague Dr Sarah Lewis, from the University of Bristol, said the research showed that levels of alcohol that are normally considered harmless can have an impact on childhood IQ.
"This is evidence that even at these moderate levels, alcohol is influencing foetal brain development," she added.
Alcohol passes easily from a mother's bloodstream to her baby via the placenta.
When ethanol alcohol enters the body, certain enzymes convert it to acetaldehyde.
Variations in genes that code for these enzymes mean that some individuals metabolise alcohol more efficiently than others.
In slow metabolisers, peak alcohol levels persist in the body for longer.
No unborn baby, whatever kind of genes it possesses, has an adult's ability to metabolise alcohol.
Dietician Catherine Collins, from St George's Hospital NHS Trust in London, said alcohol was likely to stay longer in the bodies of unborn babies with the susceptibility genes and do more damage.
She added: "What do mums take from this?
"Unfortunately it's a bit of a gene lottery.
"If your child has a particular gene profile, drinking any alcohol in pregnancy will have an effect on IQ - but, and it's a big but - your child may not have one of those identified gene defects, and so the effect is negligible."
Dr Simon Newell, from the Royal College of Paediatrics and Child Health, said: "We already know that an estimated 6,000 babies a year in the UK are born with brain damage, physical problems or learning disabilities as a result of heavy alcohol consumption by their mothers while pregnant.
"It is impossible to say what constitutes a 'safe' amount of alcohol a mother can drink as every pregnancy is different, so our advice to mothers is don't take the chance with your baby's health - drink no alcohol at all."
The research forms part of the Avon Longitudinal Study of Parents and Children (Alspac), which enrolled more than 14,000 pregnant women in 1991 and 1992.
Alspac scientists have been closely scrutinising the health and development of the women and their children since the start of the investigation.
They are currently recruiting the next generation into the study.