The official explanation is that there are organic pollutants in the consumed fish, but I think the overload of omega-3 that come from consuming fish at least 3 times is a much more likely explanation.
https://www.sciencedaily.com/releases/2016/02/160215113907.htm
"...Women who ate fish more than three times per week when they were pregnant gave birth to children with higher BMI values at 2, 4 and 6 years of age compared with women who ate fish less. High maternal fish intake during pregnancy also was associated with an increased risk of rapid growth from birth to 2 years and with an increased risk of overweight/obesity for children at ages 4 and 6 years compared with maternal fish intake while pregnant of once a week or less, the results indicate. The magnitude of the effect of fish intake was greater in girls than boys."
https://www.sciencedaily.com/releases/2016/02/160215113907.htm
"...Women who ate fish more than three times per week when they were pregnant gave birth to children with higher BMI values at 2, 4 and 6 years of age compared with women who ate fish less. High maternal fish intake during pregnancy also was associated with an increased risk of rapid growth from birth to 2 years and with an increased risk of overweight/obesity for children at ages 4 and 6 years compared with maternal fish intake while pregnant of once a week or less, the results indicate. The magnitude of the effect of fish intake was greater in girls than boys."