The amount of fish a woman eats while pregnant may affect her child’s chances of developing attention-deficit/hyperactivity disorder (ADHD).

Eating fish twice a week was linked to about a 60% lower risk of a child developing certain ADHD-like symptoms, according to research from the Boston University School of Public Health.

But the type of fish eaten is the key.

Elevated mercury levels, which can occur from eating certain types of fish, such as tuna and swordfish, were also tied to a higher risk of developing ADHD symptoms such as a short attention span, restlessness or being easily distracted.

“The really important message is to eat fish,” said assistant professor Sharon Sagiv, the study’s lead author.

Sharon Sagiv added that pregnant women should avoid “big” fish, such as tuna and swordfish, which typically contain the most mercury. Instead, they should opt for haddock or salmon.

The research was published in the Archives of Pediatrics & Adolescent Medicine.

Past studies looking at the link between mercury and ADHD – a condition estimated to affect up to 5% of school-aged children – have produced conflicting results.

Most children are diagnosed between the ages of three to seven, with boys more commonly affected.

Many people with ADHD also have learning difficulties and sleep disorders.

For the new study, the researchers followed 788 children born in Massachusetts between 1993 and 1998.

They used hair samples taken from the mothers after delivery to test their mercury levels, and food diaries to see how much fish they had eaten.

Then, once the children were about eight years old, the researchers asked their teachers to evaluate the children’s behavior to see how many exhibited ADHD-like symptoms.

After taking all of the information into account, the researchers found that one microgram of mercury per gram of a mother’s hair – about eight times the average levels found in similar women’s hair in another analysis – was linked to a 60% increase in the risk of their child exhibiting ADHD-like behaviors.

But there was no link below one microgram of mercury per gram of a mother’s hair.

The children appeared to be 60% less likely to exhibit impulsive or hyperactive behaviors if their mothers had eaten two or more servings of fish per week.