Metformin improves the endocrinopathy of polycystic ovary syndrome (PCOS), facilitates conception, appears to reduce first trimester miscarriage and gestational diabetes and does not appear to be teratogenic. The concentrations of metformin in breast milk are generally low and the mean infant exposure to metformin has been reported in the range 0.28-1.08% of the weight-normalized maternal dose, well below the level of concern for breastfeeding. No adverse effects on blood glucose of nursing infants have been reported. Metformin during lactation versus formula feeding appears to have no adverse effects on infants' growth, motor-social development and intercurrent illness during the first 6 months of life. Systematic studies have not yet been done assessing how hyperinsulinemia, polycystic ovary syndrome and metformin may affect lactation.