Advice to pregnant women about drinking is contradictory, with some guidelines recommending no alcohol at all and others suggesting the odd drink now and then is safe.

But in a study described as "hugely important" by one expert, researchers using genetic analysis of more than 4,000 mothers and children found that drinking between one and six units of alcohol a week during pregnancy can lead to lower Intelligence Quotient (IQ) scores by the time a child is eight.

"Even at levels of alcohol consumption which are normally considered to be harmless, we can detect differences in childhood IQ which are dependent on the ability of the fetus to clear this alcohol," said Sarah Lewis of Bristol University, who led the study. "This is evidence that even at these moderate levels, alcohol is influencing fetal brain development."

This study used genetic data from women and children who were part of another study called the Children of the 90s study.

Since the individual genetic variations that people have in their DNA are not connected to lifestyle and social factors, this kind of study avoids potential complications.

Most previous studies have used observational evidence, but experts say this can be misleading because, for example, mothers who drink in moderation while pregnant are typically also well educated, have good diets and are unlikely to smoke - all factors linked to higher IQ in children and which could mask any negative effects of alcohol.

A U.S. study published in July found that older, educated women are more likely to drink while pregnant.

GENES AFFECT ALCOHOL METABOLISM

This study, published in the journal PLOS ONE on Wednesday, used a new technique analyzing the genetic variants which modify the effects of alcohol exposure levels.

When a person drinks alcohol, ethanol is converted to acetaldehyde by a group of enzymes, the researchers explained.

Variations in genes that 'encode' these enzymes lead to differences in a person's ability to metabolize ethanol, so in "slow metabolizes", alcohol levels may be higher for longer than in "fast metabolizes". Scientists think fast ethanol metabolism protects against abnormal brain development because less alcohol goes to the fetus.

The mothers were asked to record their alcohol consumption at various stages during pregnancy, and one drink was specified as one unit of alcohol.

The results showed that four genetic variants in alcohol-metabolizing genes among the 4,167 children were strongly related to lower IQ at age eight. The child's IQ was on average almost two points lower per genetic variation they had.

The effect was only seen among children of women who were moderate drinkers and there was no effect evident in children of mothers who abstained during pregnancy. This strongly suggests it was exposure to alcohol in the womb that led to the difference in child IQ, the researchers 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." said Ron Gray of Oxford University, who was part of Lewis's team.

David Nutt, a professor of neuropsychopharmacology at Imperial College London who was not involved in the research, said it was "a hugely important study from the best UK cohort that can study this question".

"Even though the IQ effects are small, if at all possible women should avoid ethanol in pregnancy as it's a known toxin," he said in an emailed comment.

(Editing by Paul Casciato)

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