Mothers who eat a lot of fish during pregnancy may be putting their child at risk of rapid growth and obesity, according to research published online by JAMA Pediatrics.

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Fish oil is good for the unborn baby, but women should limit intake during pregnancy because of pollutants.

Experiences in early life play a key role in growth and development. A nutritional stressor can permanently impact physiology and metabolism, leading to health issues later in life.

But is fish not supposed to be good for you?

True, fish is an important source of polyunsaturated fatty acids, and especially omega-3.

These transfer across the placenta during pregnancy, benefitting fetal neurodevelopment and influencing the growth of fatty tissue.

In a previous study of 151,880 mother-child pairs, infants whose mothers ate fish regularly had a higher birth weight and were less likely to be born preterm.

Pollutants may disturb hormonal development

However, eating fish also increases exposure to persistent organic pollutants, some of which have been associated with disruption of the endocrine system.

Fast facts about dietary fish
  • The FDA recommend avoiding shark, swordfish, king mackerel and tilefish due to high levels of mercury
  • Pregnant women should eat 8-12 oz (2-3 servings) a week of a variety of fish and shellfish that are lower in mercury
  • Salmon, shrimp, pollock, tilapia, catfish, cod and light, canned tuna are low in mercury.

Learn more about fish oils

Scientists have suggested that the pollutants may disrupt the signaling of several nuclear receptors. This could lead to altered gene expression and influence fat metabolism, potentially contributing to the development of obesity.

In 2014, the US Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) recommended that women who are pregnant, breastfeeding or likely to become pregnant should consume a maximum of three servings of fish per week. More, they believe, could aggravate the risk of fetal exposure to methyl-mercury.

However, it remains unclear how much fish mothers-to-be should eat, and which types they should avoid to protect their child's growth and development.

Dr. Leda Chatzi, PhD, of the University of Crete, in Greece, and coauthors carried out a large-scale, multicenter, population-based birth cohort study to investigate links between maternal fish consumption and childhood growth and weight.

They analyzed records from 26,184 pregnant women and their children in Europe and the US, and they followed up the children until the age of 6 years.

The data referred to deliveries in Belgium, France, Greece, Ireland, Italy, the Netherlands, Norway, Poland, Portugal, Spain and Massachusetts - in the US - from 1996-2011.

Average fish intake during pregnancy depended on location, ranging from 0.5 times a week in Belgium to 4.45 times a week in Spain.

Consuming fish more than three times per week was considered high intake; moderate intake was more than once a week but not more than three times; low fish intake was once a week or less.

Higher BMI and obesity rates if mothers ate fish often

From birth to 2 years, 8,215, or 31%, of the children grew rapidly. At 4 years, 4,987, or 19.4%, were overweight or obese and 3,476, or 15.2%, were overweight or obese at 6 years.

Those whose mothers ate fish more than three times a week during pregnancy had higher body mass index (BMI) values at 2 years, 4 years and 6 years of age, compared with children whose mothers ate less fish.

High fish consumption was also linked to faster growth from birth to 2 years and higher rates of overweight or obesity for children at 4 and 6 years, compared with low maternal fish intake. Furthermore, girls were affected more than boys.

The authors caution, however, that the link is not conclusive.

Firstly, the available data did not distinguish between fish types, cooking methods and where the fish came from, whether from rivers or the sea. Information was also lacking about levels of persistent organic pollutants in the different locations.

Nevertheless, they say:

"Contamination by environmental pollutants in fish could provide an explanation for the observed association between high fish intake in pregnancy and increased childhood adiposity."

The researchers conclude that their findings are in line with the fish intake limit for pregnancy proposed by the US FDA and EPA.

Medical News Today reported last month that eating fish during pregnancy can contribute to healthy brain development in the fetus by providing omega-3 oils.