Women in the lowest quintile of omega-3 serum levels during first and second trimesters had 10 times higher risk of early preterm birth when compared with women in the three highest quintiles.

Women in the second lowest quintile had 2.7 times higher risk of preterm birth compared with women in the three highest quintiles.

Salmon is a great source of omega-3 fatty acids but many pregnant women are afraid of eating fish because of mercury. A 2015 American Journal of Clinical Nutrition study showed that eating omega-3 fatty acids from fish actually protects the brain from the toxic effects of mercury even in the developing fetus (which is the most susceptible to mercury effects).

There are fish with high levels of mercury, those should be avoided particularly if you are pregnant. Fish that are high in methylmercury include predatory fish such as tilefish from the Gulf of Mexico, shark, swordfish, and king mackerel.

Many of the most commonly-eaten fish are lower in mercury including wild salmon, shrimp, pollock, tilapia, catfish, and cod. I like salmon because in addition to having one of the lowest mercury contents it also has one of the highest omega-3 fatty acid contents, basically everything you want when choosing a fish to eat.

American Journal of Nutrition Study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4340059/