Pregnant women who eat salmon before giving birth could be cutting down levels of essential disease-fighting antibodies in their breast milk and putting their babies more at risk of infection, a new study revealed.

The study, published in The Journal of Nutrition, consisted of 123 pregnant women who rarely ate oily fish.  Participants were randomly assigned to eat their normal diet or to eat two portions of farmed salmon a week from 20 weeks of pregnancy until delivery. 

British scientists from the University of Reading and the University of Southampton found that while mothers who eat salmon during the latter stages of their pregnancy have an increased proportion of long-chain omega-3 fatty acids in their breast milk in the first month after birth. The researchers also had lower levels of secretory immunoglobulin-A, an important antibody that boosts immunity in babies and helps newborns combat against infection.

Researchers said that one of the main reasons breastfeeding in the first months after birth is strongly recommended by health professionals is because breast milk bolsters the immune system and helps protect babies against infection.

The team noted that expecting mothers are currently encouraged to eat several servings a week of certain kinds of oily fish known to provide high levels of omega-3 because previous studies found that mothers who eat more oily fish, like salmon, before giving birth boost levels of vital nutrients like omega-3 fatty acids, which are crucial for the baby's brain, eye and heart development.

However, they said that very little is known about the influence of eating oily fish during pregnancy on the omega-3 fatty acid content of the mother's milk, and on immune substances, like antibodies passed from mother to baby during breastfeeding.

"Pregnant women in the UK are currently advised to eat one or two servings of oily fish a week, while limiting tuna and avoiding shark, swordfish and king mackerel. This is intended to balance the need for sufficient levels of omega-3-rich food while limiting foods that might contain high levels of mercury," lead researcher Parveen Yaqoob, Professor of Nutritional Physiology at the University of Reading, said in a statement.

Yaqoob said that although results from the latest study suggested that pregnant women with diets high in oily fish passed on beneficial nutrients to their babies while breastfeeding, more research is needed to assess how the resulting lower levels of antibodies in breast milk could affect newborns.

"From this study, it is positive to note that by following the current guidance on oily fish consumption during pregnancy, women can boost beneficial nutrients to help the early growth of their babies at a crucial stage of development," Yaqoob said.

"Breast milk contains a number of ingredients which contribute to immunity, and from this work we cannot say if the reduction in one type of antibody would have any effect on a baby's health," he noted.