Among pregnant women at high risk of preterm birth, those who eat fish a few times a week may be less likely to deliver early, a new study finds.
Whether fish itself helps prevent preterm delivery is not clear, the researchers stress. But, they say, the finding is in line with the general advice that pregnant women eat up to two fish meals per week.
The study, reported in the journal Obstetrics & Gynecology, included 852 pregnant women who were at higher than average risk of preterm delivery because they had delivered early before.
Overall, 70 percent said they had averaged at least a half-serving of fish per week during the first 4 to 5 months of pregnancy. Of those women, 36 percent went on to have a preterm birth.
In contrast, that rate was 49 percent among women who ate fish no more than once a month.
In general, the researchers found, women who ate two or three servings of fish per week were about 40 percent less likely to deliver early than women who ate fish less than monthly.
There was no evidence, however, that a higher fish intake was related to any further cuts in preterm-birth risk.
"It wasn't that the more fish you ate, the better," said lead researcher Dr. Mark A. Klebanoff, of Nationwide Children's Hospital in Columbus, Ohio.
In fact, he said, the benefit may not come from fish at all.
The researchers tried to account for factors that could make fish-eaters different from other women -- like their weight, race, education levels and smoking habits.
But, Klebanoff said, there could still be something else about fish-lovers that explains the connection.
"Whether it's the fish itself or something else, we cannot say," Klebanoff told Reuters Health.
"Fish may have health benefits," he said, "but a lower risk of prematurity is not necessarily one of them."
Still, Klebanoff pointed out, the U.S. Food and Drug Administration and the American College of Obstetricians and Gynecologists (ACOG) already recommend that pregnant women eat up to two fish meals per week.
The reasoning is that fish is generally considered a healthy choice, being a source of omega-3 fatty acids. However, fish are often contaminated with traces of mercury, which could harm the developing fetal nervous system.
So the advice is for pregnant women to choose fish low in mercury -- like canned light tuna, salmon and shrimp -- and limit themselves to a couple servings per week. They should completely avoid certain fish high in mercury: namely, shark, swordfish, king mackerel and tilefish.
"Our findings are in line with the recommendations that are out there from the U.S. government and ACOG," Klebanoff told Reuters Health.
Exactly why fish intake was tied to the odds of preterm birth is not clear.
The women in this study had all been part of a clinical trial looking at whether omega-3 supplements could curb the risk of preterm birth in high-risk women. And the trial found no benefit of the fatty acid over placebo pills.
So why would fish intake, but not omega-3 supplements, be linked to a lower risk?
One possibility, Klebanoff said, is that other nutrients in fish help lower the odds of preterm birth. Another is that the trial started women on omega-3 too late: they began taking supplements between the 16th and 21st week of pregnancy.
The bottom line, according to Klebanoff, is that the findings support guidelines for women to eat up to a couple servings of low-mercury fish per week. "But you probably don't want to go over that," he said.
It's not clear whether the current findings might be true of all pregnant women -- including, Klebanoff said, those at average, rather than increased, risk of preterm delivery.