Now a large-scale new study, which looked at more than 50,000 cases in Scotland, England and Denmark over the past 80 years, has concluded that it seems likely that there is some link, although it cannot be said that flu in the mother is a cause of schizophrenia in the child.
'The effect is very small,' says Robert Kendell, professor of psychiatry at Edinburgh University and one of the authors of the report. 'This is why we had to look at such large numbers. The chances of any one woman who has flu having a schizophrenic baby is incredibly low. At most what we are looking at is a cause of 5 per cent of all schizophrenia cases. What we are really interested in is the causes of schizophrenia.'
The research began as an attempt to solve a mystery but has ended up creating a number of new ones. The initial puzzle was the well-established fact that the most common time of the year for schizophrenics to be born is in January and February. So it was suggested that this might be the result of something, say a virus, that affected pregnant women in winter. Flu was a good candidate because it also peaked in winter. Then there were the occasional epidemics, which should produce a marked effect if there were a connection.
What Professor Kendell's team uncovered was evidence of some link, but also a further riddle - it seems only to affect female children. Why, no one knows. 'If it were males it would be easier to understand,' Professor Kendell said. 'Generally they are more susceptible to harm in the womb.'
So should pregnant women at risk from flu be vaccinated? As yet the answer is another 'don't know', because doctors cannot say what causes the damage. They do not think it is likely to be flu virus itself, since - unlike rubella - it does not easily cross into the womb. They think it is more likely that it is the woman's own immune response to the virus that does the damage. This is a line of research that Professor Robin Murray of the Institute of Psychiatry in London is working on.
'We think that some women may have a gene that causes a strong antibody reaction to flu,' he says. 'Most of the time that will be very valuable, but when they are pregnant those antibodies could damage the baby's developing brain.' If this theory proves correct then vaccinating a pregnant woman against flu - and thus activating the antibodies - would be precisely the wrong thing to do.
There certainly seems to be a genetic element in schizophrenia - children are more likely to develop it in families where it already exists - and, if Professor Murray's theory is correct, it may turn out that a gene involved in schizophrenia also controls part of the immune system. It would then be possible to pinpoint those women whose babies might be damaged by flu.
Another family element appears in a study reported this month by Dr Pak Sham of the Institute of Psychiatry, who found that the children most likely to develop schizophrenia were those who had brothers or sisters four years older. If the flu connection is correct, it could also follow that there is more risk of flu in these families, with the older children bringing infection into a home where the mother is pregnant.
One element of this mystery that does seem certain is that the damage occurs in the second trimester (the fourth, fifth and sixth months). At the moment, because of the way records are kept it is difficult to be more precise, but Professor Kendell thinks it is possible that the vulnerable stage may only last a few weeks.
In the face of this theoretical uncertainty, physicians at the clinical end are counselling caution. Dr Michael de Swiet, a gynaecologist at Queen Charlotte's Hospital, west London, points out that one of the dangers of medical advances is that smaller and smaller risks are highlighted. 'All too often the main effect is that mothers end up anxious for no good reason.' Dr Andrew Dawson, an obstetrician at Cardiff College of Medicine, feels that until research has uncovered some more facts the only policy is to wait and see.
But before planning to avoid winter babies, it is worth knowing that another study found the greatest number of achievers - as defined by a mention in Encyclopaedia Britannica - were born in February.
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