An excerpt from the lactation page on Dr. Holub's website:
DHA (docosahexaenoic acid, 22:6n-3) is a physiologically-essential essential nutrient and a key omega-3 fatty acid needed in high levels in the brain and retina (eye) for optimal neuronal functioning (learning ability, memory) and visual acuity, respectively.The research suggests that lactating women with a low dietary intake of fish should supplement with fish oil. I took fish oil during both pregnancies but stopped once my babies were born. This is good motivation to do keep up the habit!
For breast-fed infants, their only source of nutrition (incl. DHA) for growth and development is their mother's milk. The amount of DHA in the diet is a major factor determining how much DHA appears in breast milk for the baby to consume for health. Since fish is by far the predominant food source of dietary DHA, and since fish is consumed at a very low rate (approximately one serving every 10 days), the level of DHA in North American breast milk is very low. For example, Health Canada has reported that DHA represents an average of only 0.14% of the total fat in breast milk. This low level reflects the low dietary intake of DHA during lactation of approximately only 80 mg/day.