'Tis the Season

Reports on the outcome of a research on birth defects among babies born in different seasons of the year. Months with high incidence of autistic babies delivered; Factors causing defects including biological and environmental conditions.

By PT Staff, published on November 1, 1995 - last reviewed on June 9, 2016


If you were born in December, you've probably griped about your birthday falling near Christmas or Hanukkah. But here's some news that might put you m the holiday spirit: that birthday cut your odds of being autistic in half.

It seems that people born in some months are less likely to wind up with certain mental and cognitive disorders. Take autism. In Israel, 17 percent of babies are born in November and December-but only 8.5 percent of autistic kids. March and August, however, are danger periods, accounting for a third of autistic births, report Israeli researcher Yoram Barak, M.D., and colleagues in the American Journal of Psychiatry (Vol. 152, No. 5).

The season-of-birth effect doesn't stop at autism. Schizophrenia, mental retardation, and Alzheimer's disease may all have high-risk birthdays.

Don't blame the zodiac. The likely culprits are biological and environmental factors that wreak havoc at particular times of the year. "Viruses are the leading contender," says E. Fuller Torrey, M.D., guest researcher at the National Institute of Mental Health. Other suspects include extreme seasonal temperatures and winter vitamin deficiencies.

Whatever the cause, it need not operate in the actual month of birth. It just coincides with crucial periods of early development. So the increase in autism among folks born in March might reflect exposure to winter viruses late in pregnancy—or to some late-summer factor that took place right after conception. "You may even be dealing with something that's affecting children in the first three months after birth," says Torrey.

Most birth-month fluctuations in these disorders are small: schizophrenia is only 10 percent more common among the winter-born. But figuring out why seasonal variations occur could help identify causes of these disorders in everyone. Viruses, after all, invade us year round; they're just more successful in winter. So it's possible the same agent responsible for the rise in autism among the March-born also causes the disorder in those with low-risk birthdays.


January Schizophrenia, Alzheimer's

February Schizophrenia, Alzheimer's

March Autism, Schizophrenia,




June Schizophrenia

July Autism, Schizophrenia

August Autism Schizophrenia

September Schizophrenia


November Schizophrenia

December Schizophrenia Autism