A study of more than a million people in Sweden has found lower birth weight is linked to a small but significant increase in the risk of mental health problems, including depression.
The study, led by Erik Pettersson from the Karolinska Institute in Stockholm, extracted the birth records of 546,000 sibling pairs born between 1973 and 1998 then traced their psychiatric history through to 2013.
The aim was to address two abiding questions in the existing literature which already, in fact, links impaired foetal growth to mental health conditions including depression, anxiety and bipolar disorder.
The first question is whether the link is confounded by other things in the affected family. Genetics or poverty, for example, could cause both low birth weight and mental illness and thus be the real culprits.
The second issue is the big overlap between psychological problems. Many people with depression also have anxiety, for example, leading some studies to suggest that a “general” factor, or broad predisposition to mental illness, best explains risk. These researchers, however, wanted to drill down and see if foetal growth changed the risk for specific psychiatric conditions.
On the first question they found, across the board, that lower birth weight increased risk for nine conditions including depression, post-traumatic stress and bipolar disorder.
Then they looked to see if the same effects held up within sibling pairs. Because siblings share genes and grow up in a similar environment, one that might include malnourishment, for example, this check can show if shared factors actually account for the results.
The new analysis put quite a different spin on things.
After controlling for family connection, higher birth weight was protective against just four conditions: depression, obsessive-compulsive disorder, attention-deficit/hyperactivity disorder (ADHD), and autism.
On the “general” versus “specific” question, the team found a one kilogram increase in birth weight reduced both general predisposition to psychological illness and specific risk for “neurodevelopmental” disorders, which include autism and ADHD.
What, then, might explain the relationship?
“One speculation is that reduced foetal growth compromises brain development during a critical period, which in turn slightly increases the risk not only for neurodevelopmental disorders but also for virtually all mental health conditions,” the authors write.
They stress the effect sizes were small in this study; higher birth weight reduced the odds of depression by just 5%.
“Nevertheless,” they conclude, “given the sheer prevalence of mental health conditions, combating maternal malnourishment and improving prenatal care still might influence a significant number of cases.”