A study has found that mothers who work in jobs where they are frequently exposed to toxic chemicals or experience high stress have higher chances of having a child with autism spectrum disorder (ASD).

Researchers of the new study, published online by BMJ’s Occupational and Environmental Medicine journal, examined data on mothers’ occupations held before conception, during pregnancy and during infancy to investigate any impact on the neurodevelopment of any offspring.

However, as this is an observational study, no firm conclusions can be drawn about cause and effect.

The study found the chances of having a child with ASD were increased for mothers who were employed before conception up to infancy in ground transportation, public administration and in the military.

Working in military or defence occupations before or during pregnancy increased the odds of a child being diagnosed with ASD by 59 per cent, the study found.

Researchers suggest working in defence occupations may expose mothers to hazards such as lead from artillery handling and training exercises, exhaust fumes and industrial solvents.

Mothers who worked in ground transportation, where they may be exposed to combustion products, had a 24 per cent greater chance of having a child with ASD.

Researchers said mothers employed in ground and air transportation may be exposed to exhaust fumes and particulate matter.

The chances of having a child with ASD was increased by 59 per cent if mothers worked in the judicial sector before conception and during pregnancy.

The researchers suggest stress at work may contribute to maternal fatigue and discomfort, which may adversely impact child neurodevelopment such as through increased inflammation during pregnancy.

The study did not find any link between ASD and the mother working in agriculture where they may have been exposed to pesticides.

The associations were observed for occupations held one year before conception, during pregnancy and during infancy, although associations were weakened during the child’s infancy.

This study included a total of 110,234 participants. They obtained data for 1,702 cases of ASD in children born between 1973 and 2012 in Denmark. These cases were matched to 108,532 non-autistic controls based on sex and birth year.

Some 70 per cent of the cases of ASD and the controls were male and almost half (46 per cent) were born in the 1990s.

The mothers’ employment histories were obtained from the Danish Pension Fund Registry and grouped into seven occupational categories.

In their analysis, researchers also took potential confounders into account. This included the mothers’ age, history of neuropsychiatric disorders, whether they smoked during pregnancy and socioeconomic status.

Researchers said previous studies have shown potential associations between certain maternal occupations and ASD in the child. However, the results are inconsistent as small study samples were used or previous studies relied on self-reported data and many occupational categories were not evaluated.

A limitation of this study was that the occupations were based on industry categories and did not include detailed information on job titles or tasks.

It may not be feasible to link the study results to populations outside of Denmark. Researchers suggest that future studies should investigate specific exposures, both alone and jointly, to see which have the greatest impact on the likelihood of ASD in offspring.

The researchers concluded: “Maternal occupations with frequent exposure to toxicants and combustion products, as well as high-stress occupations, could contribute to neurodevelopmental risk.”