Doctors are eager to find ways to prevent asthma, a chronic disease that causes wheezing, coughing and breathing trouble, and that sends many families to the emergency room again and again.

The incidence has more than doubled in developed countries in recent decades. More than six million children in the United States have asthma, according to the Centers for Disease Control and Prevention, as do more than 330 million children and adults worldwide, according to the Global Asthma Network.

Dr. Bisgaard said it was not possible to tell from the study whether pregnant women could benefit from simply eating more fish. Pregnant women are generally advised to limit their consumption of certain types of fish like swordfish and tuna because they contain mercury. But many other types are considered safe, especially smaller fish like sardines that are not at the top of the food chain and therefore not likely to accumulate mercury and other contaminants from eating other fish.

The results were published in The New England Journal of Medicine. The scientists bought fish oil from a company that makes it, but they said the company had no role in the study. The research was paid for by the Danish government and private foundations.

An editorial in the same journal by an expert who was not part of the study praised the research, saying it was well designed and carefully performed. The author of that editorial, Dr. Christopher E. Ramsden, from the National Institutes of Health, said the findings would help doctors develop a “precision medicine” approach in which fish-oil treatment could be tailored to women who are most likely to benefit.