It could be the case that eating fish is better than taking fish-oil supplements or that women who opt to eat fish are generally healthier and engage in other health-promoting behaviors, Dr. Oken said.

WSJ: Fish Oil Provides Little Benefit for Pregnant Women

The results are "disappointing, " said Emily Oken, a professor of population medicine at Harvard Medical School and Harvard Pilgrim Health Care Institute, who wrote an editorial to accompany the paper but wasn't involved in the study.

WSJ: Fish Oil Provides Little Benefit for Pregnant Women