Pregnancy complications may increase autism risk

Embargoed until 01 July 2009

Complications during pregnancy may increase the risk of having a child with autism, according to American researchers.

The team reviewed 64 studies of prenatal risk factors for autism. It is the first time a meta-analysis of the relationship between pregnancy-related factors and risk of autism has been carried out. The analysis is published in the July issue of the British Journal of Psychiatry.

Over 50 prenatal factors were examined. The researchers found that the factors most strongly associated with an increased autism risk are:

• Being born to an older mother or father.
• Having a mother who was born abroad.
• Having a mother who experienced bleeding during pregnancy.
• Having a mother who experienced gestational diabetes.
• Having a mother who used medication during pregnancy.
• Being the first born - or later born in families where there are three or more children.

The researchers put forward possible explanations for these risk factors. For example, increased maternal age may be associated with autism because of a higher risk of chromosomal abnormalities in eggs.

Mothers who are born in another country may not have natural resistance to infections in the country where they give birth, which may increase the risk for autism. Moving to another country may also put women under stress, which could increase their chances of having a child who develops autism.

Bleeding during pregnancy, gestational diabetes and medication use are also associated with increased autism risk. Bleeding can cause foetal hypoxia (lack of oxygen to the brain of an unborn child). Women who develop diabetes during pregnancy experience hormonal and metabolic changes, which may affect their baby’s health and development. Foetal development may also be affected by some medications which can cross the placenta during pregnancy.

The association between birth order and autism risk is unclear. However, children with autism are more likely to be the first-born in families with only two children. In larger families with three or more children, they are more likely to be born later. It is possible that parents decide not to have more children after one has developed autism.

The researchers said there was “insufficient evidence” to point to any one prenatal factor as being particularly significant. However, writing in the British Journal of Psychiatry, they said: “There is some evidence to suggest that exposure to pregnancy complications in general may increase the risk of autism.”

References:

Gardener H, Spiegelman D and Buka SL (2009) Prenatal risk factors for autism: comprehensive meta-analysis, British Journal of Psychiatry, 195: 7-14