Leptin was first discovered in human breast milk in 1997. It can be transferred from maternal serum and is produced by a lactating breast (source). It is found in its highest levels in colostrum during the first few days of life and slowly decreases during the following six months (source).
High levels of leptin in early lactation have been shown to provide protection to infants from excess weight gain, yet they still remain within normal weight ranges. It could play an important role in breastfed infants showing lower rates of obesity when compared to formula fed infants (source).