The season in which a baby is born apparently influences the risk of developing mental disorders later in life, suggests a large new study.

The season of birth may affect everything from eyesight and eating habits to birth defects and personality later in life. Past research has also hinted the season one is born in might affect mental health, with scientists suggesting a number of reasons for this apparent effect.

"For example, maternal infections — a mother may be more likely to have the flu over the winter. Does this increase risk?" said researcher Sreeram Ramagopalan, an epidemiologist at Queen Mary University of London. "Or diet. Depending on the season, certain foods — fruits, vegetables — are more or less available, and this may impact on the developing baby."

"Or another key candidate is vitamin D, which is related to sunshine exposure," Ramagopalan added. "During the winter, with a lack of sunshine, mums tend to be very deficient in vitamin D."

However, this effect appears very small, and since past studies only looked at several thousand people at a time, there was a chance the link between birth month and later mental health might only be a statistical illusion. Also, prior research often pooled data from different nations, complicating analysis, since population trends can vary substantially between countries.

To pin down whether or not there was a link between seasons and the mind, Ramagopalan and his colleagues analyzed a very large number of births, all from the same country.

The scientists investigated whether the risk of schizophrenia, bipolar disorder and recurrent depression was influenced by month of birth in England. This included nearly 58,000 patients with the disorders and more than 29 million people from the country's general population. [10 Controversial Psychiatric Disorders]

The researchers found that all the mental disorders they looked at showed seasonal distributions. Schizophrenia and bipolar disorder had statistically significant peaks in January, and significant lows in July, August and September. Depression saw an almost significant May peak and a significant November deficit.

"This result is further confirmation of seasonal variations in births of those later diagnosed with mental diseases," said William Grant at the Sunlight, Nutrition and Health Research Center at San Francisco, who did not take part in this research. "This implicates conditions during pregnancy. The two most likely factors are vitamin D status and temperature."

The differences in risk between the disorders could be a result of different factors, or the same factor being important at different periods of pregnancy, Ramagopalan speculated. For example, the same risk factor — say, vitamin D levels — could be important in the third trimester for schizophrenia and bipolar disorder and the second trimester for depression.

"The major implication is that once we understand the cause of these effects, then we can intervene in terms of disease prevention," Ramagopalan told LiveScience.

Factors other than prenatal ones might be involved as well. For instance, children born late in the year may be relatively immature compared with older classmates, and thus do less well academically and socially, which might cause mental stress. "Further, we did not have details on socio-economic status or ethnicity, which may confound our results," Ramagopalan said.

Future research to understand the causes of these effects "would require large birth cohort studies to follow individuals over time," Ramagopalan said.

Ramagopalan and his colleagues detailed their findings online April 4 in the journal PLoS ONE.