According to a study, prenatal exposure to the virus may reduce the IQ of the offspring measured 20 years later by three to seven points.
The claims are made in the medical journal, Annals of Neurology. The editorial says: "These findings suggest that a bout of flu early in pregnancy may affect the cognitive ability of the mature subject evaluated 20 years later. This finding will no doubt engender considerable controversy."
Mental abilities in adulthood are known to be linked to normal brain development during gestation. The effect of maternal alcohol consumption on the mental ability of the child has been shown, and malnutrition, maternal exposure to cigarette smoking and illicit drugs are also suspected to be detrimental to foetal brain development.
But it is estimated that in 25 to 35 per cent of cases, particularly in those with mild mental defects, the cause is not known, and its suspected that some of these may be related to other kinds of environmental exposure.
Two reports due to appear in the journal look at the possible affects of the flu virus on the developing brain.
A research paper from the University of Oslo and the Norwegian Institute of Public Health, and an editorial from doctors at the Université Paris Sud, centres on an outbreak of Hong Kong flu in Norway. The main outbreak occurred between November 1969 and January 1970, and overall, 15 to 40 per cent of the population are thought to have been affected.
The study involved comparing birth dates before during and after the epidemic - from 1967 to 1973 - with IQ in 200,000 men whose IQ was tested two decades later. Conscription in Norway meant that all young men were tested at the age of 20.
Results show that compared to all other years, being born in 1970s was associated with a lower intelligence score .
After taking into account a range of factors, including parental education and the linear trend of increasing IQ scores over birth years, the young men who were born between July and October 1970 - six to nine months after the main outbreak of the Hong Kong flu - were found to have a significantly lower score on mental tests than young men born during any other period, including the same months in previous and subsequent years.
"This is, to the best of our knowledge, the first report of a possible association between prenatal exposure to an influenza virus epidemic and the mean level of intelligence in the general population," say the authors from the University of Oslo.
The researchers, who say it is realistic to assume that 20 per cent of the men who were born in the period July to October 1970 were exposed to the virus during the first trimester, use the results to estimate that flu during pregnancy may decrease the IQ of the offspring by three to seven points as measured on a standard IQ scale.
The researchers say that exposure during the first three to four months of pregnancy seems to have had the strongest impact on intelligence scores, although they say they cannot rule out the possibility of effects later in the pregnancy.
Just how exposure to the flu virus could have such an effect is not known. The researchers say it is unlikely that variations in known determinants of intelligence, such as genes, parental care and stimulation, nutrition, and education, could explain the relatively low intelligence scores of the men who were born during a few consecutive months in 1970.
Possibilities include the idea that maternal infection during pregnancy could have an effect on the foetal brain, possibly through compounds linked to inflammation.
Commenting on the report, Dr Alan Hay, director of the London-based World Influenza Centre, said: "I think such an association is very difficult to tie down. There have been suggestions of links between flu and long term neurological consequences but most of these have not really evolved into something where there has been a clear causal link established. Many remain anecdotal.
"The study does not demonstrate a direct link between flu infection and IQ score. It would be unfortunate to imply from this that infection by swine flu may have long term neurological outcomes, especially as the vast majority of infection results in mild illness."