By Gene Emery

(Reuters Health) – A large Australian study is debunking the idea that fish oil capsules can lower the odds of premature delivery or raise the chances of late-pregnancy complications.

The study found virtually-identical rates of preterm delivery among 2,734 pregnancies where women were taking fish oil capsules daily and among 2,752 pregnancies where the mothers-to-be were taking vegetable oil capsules with only trace amounts of the n-3 long-chain polyunsaturated fatty acids believed responsible for fish oil’s health benefits.

More babies in the fish oil group were very large for their age than in the control group, but the researchers behind the study said that finding might have been a statistical fluke.

“The bottom line is, blanket supplementation will not be effective. It’s not that simple,” coauthor Karen Best, a research fellow at the South Australian Health and Medical Research Institute in North Adelaide told Reuters Health in a telephone interview.

Whether the supplements will help certain groups of women remains an open question. “We need to know who may or not benefit. That’s the next step,” she said.

The capsules were given out before the 20th week of pregnancy. Babies born before the 35th week were considered preterm; such babies account for most of the newborn deaths and childhood disability seen in preemies.

Previous studies have shown that the less fish consumed during pregnancy, the higher the rate of preterm delivery. The World Health Organization already recommends that pregnant women consume 300 mg of omega-3 fatty acids per day, but women in the U.S. typically consume less than a third of that level.

Best said about 80% of pregnant women take some type of prenatal vitamin product, many of which contain some degree of fish oil.

The study, known as ORIP, had women in the fish oil group taking 900 mg daily of omega-3 fatty acids – contained in three 500-mg fish-oil capsules – until their 34th week of pregnancy. The vegetable oil placebo capsules looked identical. The researchers tried to use a small amount of tuna oil to make the placebo capsules taste the same as the fish oil capsules, but many volunteers said they could tell which group they were in.

Early preterm delivery occurred in 2.2% of the fish oil pregnancies and 2.0% of the vegetable oil pregnancies, an insignificant difference, the study team reports in The New England Journal of Medicine.

There was a 30% greater chance that a child born to a mother taking fish oil would be very large for gestational age, although the actual number of babies who fell into that category was small. The rates were 5.2% in the fish oil group and 4.0% in the vegetable oil group.

Although the fish oil babies tended to be larger, that did not increase the risk of cesarean-section, the need for oxygen or other complications during delivery, which Best said was good news.

The rates of miscarriage, preeclampsia and diabetes for the mother were comparable in the two groups.

The most significant side effect: women taking the fish oil capsules were more likely to report burping.

Best and her colleagues note that it’s still possible fish oil might benefit the babies of pregnant women who have low levels of omega-3 fatty acids.

SOURCE: The New England Journal of Medicine, online September 11, 2019.