This study is designed to test whether an alternative medicine treatment, fish oil, will prevent depressive symptoms in pregnant and postpartum women who have been found to be at risk for depression. Epidemiologists have observed that people who live in countries where people on average eat a diet high in fish have a lower risk of depression than people who live in populations that eat less fish. Postpartum depression is also less common in these countries. The omega-3 fatty acids found in fish oil are thought to be responsible for this beneficial effect of eating fish. The two major omega-3 fatty acids found in fish are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). DHA is an essential building block of the brain and nerve tissue. EPA may act to optimize the electrical signals between nerve cells and brain cells and may help the immune system to function well.

Some researchers have treated people who are already suffering from depression with fish oil. Some of these studies have shown a benefit for the fish oil treatment and others have not. These studies have tested EPA and DHA alone and in various combinations. Currently, it is not known whether EPA or DHA is more effective in preventing and treating depression. Some of the researchers involved in this study have learned how to identify mothers who are most at risk for developing depression during and after pregnancy. This study is designed to learn whether EPA-rich and DHA-rich fish oil supplements will prevent depressive symptoms in women who are at risk to develop depression.