The two key Omega-3 essential fatty acids in fish oil are eicosapentaenoic acid (EPA) and docohexaenoic acid (DHA), the latter being a major component of brain tissue. Perhaps the old wives tales were right in calling fish "brain food."
Fish oil decreases anger, anxiety, and depression and increased vigor — while improving various types of cognitive and physiological functions, and mood. It fights dry eyes and may help promote heart health while decreasing stroke risk.
Fish oil intake (containing Omega-3 fatty acid) during pregnancy may boost a baby's growth rate. Research suggests the benefits may extend throughout a child's life — helping prevent asthma, as well as decreasing the risks of bipolar disorder and cancer (in females). It also helps prevent postpartum depression. Many consider fish oil deficiency to be the most important nutritional deficiency in pregnancy, but do not want to recommend increasing fatty fish for fear of their having high mercury levels. Taking pure and mercury free fish oils avoids this problem.
If you would not eat a piece of fish that tastes like the oil, then the oil is rancid and don't use it.