According to research, the supplements brought the risk from 23.7 per cent among mothers in the placebo group who took 2.4 grammes of olive oil daily down to 16.9 per cent in the women who got the fish oil capsules. That's a 30.7 per cent reduction during the first three years of life.
The real benefit seemed to be exclusively among the children whose mothers started out with low levels of the two key ingredients in fish oil - eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) - that may make them more vulnerable to the inflammation and heightened immune system response that is a factor in asthma and related conditions.
Kathleen Melanson, a professor of nutrition sciences at the University of Rhode Island in Kingston, who was not involved in the research said, "If it weren't for the effects in that subgroup, then the results would have not been statistically significant."
Among the Danish women with low EPA and DHA levels, the rate of asthma and wheezing in their children was 17.5 per cent when the women took fish oil during pregnancy versus 34.1 per cent when they took the placebo oil .
One in five young children are affected by asthma and wheezing disorders. In recent decades, the rate has more than doubled in Western countries.
Previous research has shown that those conditions are more prevalent among babies whose mothers have low levels of fish oil in their bodies. The new large-scale test, reported in The New England Journal of Medicine, is the first to see if supplements can actually lower the risk .
Chief author Dr Hans Bisgaard of the University of Copenhagen stated that women can eat fish to get the same benefit but they really need to be very fond of fish to get sufficient amounts through diet to have the effect seen in the study.
Dr. Christopher Ramsden of the National Institute on Aging said, the 2.4 gramme dose "was approximately 15 to 20 times as high as the average US intake from foods."
He further added, "Before these findings can be applied to clinical practice, it is therefore imperative to ensure that this dose had no adverse effects on behavior, cognition, or other long-term outcomes. Future work is also needed to determine whether lower doses are effective and whether these results can be replicated in other populations."
Melanson told media that the dose used in the study was very high. The World Health Organisation recommends no more than three grammes per day, in part because excessive amounts can increase the risk of bleeding, lower blood pressure, and interact with medicines or vitamins A, D and E.
"Too much of a good thing is never a good thing," she said.
The Bisgaard team said, "It is possible that a lower dose would have sufficed."
But the supplements didn't seem to affect the odds of a baby or toddler developing the skin condition eczema, or an allergy such as a reaction to milk or egg products, or a severe asthma attack.
The researchers calculated that 14.6 women would need to be treated to prevent one case of asthma or persistent wheeze. Among women with the lowest levels of EPA and DHA to start with, only 5.6 would need to be treated.