A giant meta-analysis incorporating over 400 different studies and 1.5 million people has examined the effect of diet during pregnancy on a baby's risk of developing allergies or eczema. The comprehensive study concluded that a mother consuming fish oils or probiotics while pregnant can have markedly positive effects on her baby's well-being.

"Food allergies and eczema in children are a growing problem across the world," says Robert Boyle, lead author on the research. "Although there has been a suggestion that what a woman eats during pregnancy may affect her baby's risk of developing allergies or eczema, until now there has never been such a comprehensive analysis of the data."

A team from Imperial College London examined a broad assortment of different studies examining various dietary interventions during pregnancy, lactation, or the first year of life and their associations with a future risk of allergic or autoimmune disease in the baby. Positive health affects were seen as a result of a pregnant mother's consumption of both fish oil and probiotics.

Looking at probiotics, primarily the bacterium Lactobacillus rhamnosus, the research found that when taken in capsule form, mostly during late pregnancy and breastfeeding, a child's risk of developing eczema was reduced by 22 percent. The study notes that any mechanism that could explain this correlation is still unknown.

In terms of fish oil, the study found strong evidence that omega-3 fatty acid supplements taken by a mother during pregnancy resulted in a 30 percent reduction in egg allergies by age one and a 38 percent reduction in peanut allergies. The researchers do note that the reduction in peanut allergies is based on less overall data than the conclusion on egg allergies.

While omega-3 intake provided benefits, omega-6 fatty acids did not show any effect on a child's allergy risk, and the research notes that all but one study used fish oil supplements as opposed to eating oily fish. Moderation in fish consumption for pregnant mothers is still cautioned due to mercury traces found in many species.

The ultimate recommendations from the study are for daily probiotic supplements to be taken by pregnant mothers no earlier in a pregnancy than 36 weeks and through the first six months of lactation. Fish oil supplements should be taken from no earlier than 20 weeks into a pregnancy and through to around four months of lactation.

Interestingly, the study found that other dietary interventions yielded no association with increased risk of allergies or autoimmune diseases in a baby. This included examining timing of solid food introduction in a baby, maternal allergenic food avoidance, and many other specific vitamin, mineral, fruit, or vegetable interventions.

The researchers strikingly conclude in the paper that, "Taken together, our findings suggest that while infant diet may influence immune development through allergen-specific mechanisms, maternal diet during prenatal life and lactation may have broader effects on the developing immune system."

The study was published in the journal PLOS Medicine.