Babies born prematurely are at a much greater risk for developing severe mental disorders including psychosis, bipolar disorder and depression, according to a new study.

Researchers found that individuals born very prematurely (less than 32 weeks) were three times more likely to be hospitalized with a psychiatric illness at age 16 and older than those born full-term.

The scientists believe the increased risk may be due to small but critical differences in brain development if the child is born before the full 40-week gestation period.

The risk varied with each condition. Psychosis was two and a half times more likely for premature babies, severe depression three times more likely, and bipolar disorder 7.4 times more likely for those born before 32 weeks.

Researchers also found smaller but significant increased psychiatric risks for infants born only moderately early, between 32 and 36 weeks.

“Since we considered only the most severe cases that resulted in hospitalization, it may be that in real terms this link is even stronger,” said Chiara Nosarti from the Institute of Psychiatry at King’s College London, who led the research.

She stressed, however, that “the majority of individuals who are born prematurely have no psychiatric or cognitive problems, are absolutely healthy and well functioning.”

Overall, the disorders affect between one and six percent of the general population, she said.

Nosarti worked with researchers at the Karolinska Institute in Sweden and analyzed data from 1.3 million medical records in Sweden between 1973 and 1985 to find all those admitted to a hospital with their first episode of a psychiatric disorder by 2002.  They then looked back to see if these people had been born either very or moderately prematurely.

“The strongest association we found was to mental health disorders known to have a strong biological basis, such as bipolar disorder,” Nosarti said.

She said the results suggest that slight alterations in the developing brain of those born early may play an important role in mental health in their later years.

Earlier research has shown that preterm babies are at a greater risk for various health and developmental problems, but this study was the first to look in detail at the association between severe psychiatric disorders and premature births.

A United Nations-backed report in May reported that 15 million babies were born prematurely in 2010 and that rates of the phenomenon are rising around the world partly due to advances in medicine which allow even extremely premature babies to survive.

When asked whether this increase could be partly responsible for similar rises in mental illnesses and neurodevelopmental disorders such as autism, Nosarti said a small portion of these may be explained by premature births.

The research found that approximately six percent of people with severe depression and six percent of those with psychosis were born preterm, as were around 11 percent of people with bipolar disorder.

The study is published in the Archives of General Psychiatry.

As first appeared in Psych Central, 3 June 2012