Women drinking a glass or two of wine during pregnancy increase the risk of the child having lower IQ, says a new study.

The study was conducted by researchers from University of Oxford and Bristol University. They looked at the IQ score of more than 4,000 children and compared it with the amount of alcohol consumed by the mother during her pregnancy.

The children's IQ was tested when they were aged eight, using a shortened version of the Wechsler Intelligence Scale for Children.

Now, recent research has shown that moderate alcohol consumption is okay for mums-to-be based on observational studies. But, the new study was based on genetic analysis of the test subjects and it has found that even light drinking by the mother can be bad for the child's brain.

According to the study, four genetic variants in alcohol-metabolizing genes were strongly related to lower IQ in the children at age eight.

On average, a child's IQ dropped by 2 points with each genetic variation.

Ethanol is converted to acetaldehyde by a group of enzymes in the body. Genetic variations are responsible for making people fast metabolizers or slow metabolizers. The faster the metabolism of ethanol in the body, the lesser is its harmful effect in the fetal brain, researchers say.

"Our results suggest that 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 foetus to clear this alcohol. This is evidence that even at these moderate levels, alcohol is influencing foetal brain development," said Dr Sarah Lewis, the report's main author, according to a press release.

The study analyzed alcohol consumption of pregnant women who were moderate drinkers. By conducting the analysis using techniques like the Mendelian randomization, they were able to directly link alcohol consumption to lower IQ levels.

"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 good reason to choose to avoid alcohol when pregnant," said Dr Ron Gray from the University of Oxford who led the research.

The study is published in the journal PLOS ONE.

A recent study had found that heavy alcohol use during pregnancy shrinks the child's brain. According to recent Centers for Disease Control and Prevention (CDC) survey, one in 13 pregnant women in the U.S. admit to drinking alcohol, with many going on binge drinking.