Expectant moms who take fish oil supplements may not be doing much to sharpen their babies’ vision, a new study suggests.

The findings fly in the face of some earlier research that had suggested docosahexaenoic acid (DHA), which is found in fish oil, improves vision in preterm babies who are given supplements in their first few months of life.

DHA is an omega-3 fatty acid involved in brain and visual development. Since the substance passes through the placenta primarily later in pregnancy, preemies miss out on much of their prenatal supply.

So extra DHA after birth might help make up for that.

In the new study, Australian researchers looked at whether prenatal fish oil helps improve vision in full-term infants.

They tested visual acuity in 185 4-month-olds whose mothers had been randomly assigned to either take DHA-rich fish oil capsules or a placebo (vegetable oil capsules) every day, from mid-pregnancy until delivery.

Overall, there was no benefit of DHA on infants’ vision, the researchers report in the American Journal of Clinical Nutrition.

It’s not clear why the supplements didn’t have an impact, despite the benefits seen in preemies given DHA after birth.

But one reason may be that preemies need the extra DHA, whereas full-term babies get all they need for normal visual development while still in the womb, according to Dr. Maria Makrides, the lead researcher on the study.

“I think that if women are well nourished and have a good, varied diet, then supplementation with DHA during pregnancy to enhance visual development for their unborn baby is not necessary,” Makrides, of the Women’s and Children’s Health Research Institute in North Adelaide, Australia, told Reuters Health in an email.

Researchers are still studying whether there might be other benefits for infants’ development. But studies so far have come to mixed conclusions.

In a study published last year, Makrides’ team found no evidence that fish oil during pregnancy boosted babies’ cognitive and language skills at the age of 18 months (see Reuters Health story of October 19, 2010).

Makrides said that this and other studies are pointing out the fact that full-term babies born to well-nourished moms generally do well developmentally - and it may be hard to improve upon that with DHA supplements.

There may be other benefits of fish oil during pregnancy. Some studies have suggested that it can curb the risk of preterm birth, for example, but the jury is still out on that question.

In general, experts recommend that pregnant women strive for 200 milligrams of DHA per day. Some prenatal vitamins now carry the fatty acid, which is also present in fish - especially fatty ones like salmon, mackerel and tuna.

However, since fish can be contaminated with mercury, doctors advise pregnant women to limit themselves to two fish meals per week. They should also choose fish that have omega-3s but are likely to have low mercury levels, such as salmon, canned light tuna and shrimp.

Certain fish - shark, swordfish, king mackerel and tilefish - should be completely avoided during pregnancy, because they can have high mercury levels.

Makrides said her team is continuing to follow the children in this study to see if fish oil during pregnancy makes any difference in cognitive and language skills at age four.

As for visual development, if fish oil has any benefit for full-term babies, it would be most evident early in life.

“By the end of infancy,” Makrides said, “it would be much harder to find differences between groups, as visual development would be well advanced.”

SOURCE: American Journal of Clinical Nutrition, June 2011.