From the article:

A particularly salient finding was that the preventive effect of fish oil was driven almost exclusively by children of the prespecified subgroup of mothers with low baseline blood levels of EPA and DHA (relative reduction in risk of persistent wheeze and asthma, 54%; P=0.011). Effects were most evident in children of mothers with a variant of the gene encoding fatty acid desaturase that is associated with low ability to produce EPA and DHA from their dietary precursor, alpha-linolenic acid.

[…]

Although these results are highly promising, a note of caution is warranted. The dose of EPA plus DHA provided in this trial (2.4 g per day) was approximately 15 to 20 times as high as the average U.S. intake from foods.