• Scientific Evidence
• Helpful actions - what can you do?

Large studies of individuals in the 1970s showed that people who were diagnosed with mania were more likely to be born during the winter months. Later, this association was replicated by E. Fuller Torrey et al., in a study of 18,021 subjects with bipolar disorder and healthy controls residing in the same states (1996). The authors found that births during December, January, February, and March had a 5-6% excess of individuals who were diagnosed with bipolar disorder. Other months showed a lower incidence of bipolar births. According to Torrey (see Surviving Manic Depression, p. 85), other small studies carried out in different countries have found a similar association.

A review article of environmental risk factors for bipolar disorder (Tsuchiya 2003) found that six out of nine studies examining this phenomenon found a correlation between winter births and increased incidence of bipolar diagnosis. However, it is important to note that the months identified in each of the studies (that is, which months contained the excesses of bipolar births) were not exactly the same in every study. Tsuchiya et al. concluded that the evidence supporting this association was suggestive, but inconclusive.

There is also a correlation between winter birth and increased risk of schizophrenia or schizoaffective disorder.

The cause of these correlations are not known. Hypotheses include: genetic factors that make babies more likely to survive in certain seasons, seasonal variations in toxin or infection exposure (this is more likely for schizophrenia, given the correlation between maternal infections during pregnancy and schizophrenia risk), nutritional deficiencies during certain months of pregnancy, or seasonal differences in light that could affect internal chemistry. Scientists have noted that certain brain chemicals that regulate mood (esp. serotonin) are affected by light levels, and that some individuals may be more susceptible to light variations than others due to genetic or biological factors.

Helpful Actions:

Although it is still very unclear why individuals born from Dec-March are slightly more likely to develop bipolar disorder, prospective parents who know that bipolar disorder or other psychiatric diseases run in their families may want to try and plan for a birth outside of these winter months.

Scientific Studies:

• Risk factors in relation to an emergence of bipolar disorder: a systematic review. Bipolar Disord. 2003 Aug;5(4):231-42. Review.
• Seasonality of birth in patients with mood disorders in Japan. J Affect Disord. 2000 Jul;59(1):41-6.
• Seasonality of births in schizophrenia and bipolar disorder: a review of the literature. Schizophr Res. 1997 Nov 7;28(1):1-38. Review
• Birth seasonality in bipolar disorder, schizophrenia, schizoaffective disorder and stillbirths. Schizophr Res. 1996 Sep 18;21(3):141-9.
• Season of birth and bipolar disorder. Am J Psychiatry. 1993 Mar;150(3):526-7. No abstract available.
• Mental disorder and season of birth: a national sample compared with the general population. Br J Psychiatry. 1974 Jan;124(578):81-6. No abstract available.