What are the risk of smoking marijuana and breastfeeding my baby?

Minimize or avoid: use of marijuana while breastfeeding especially if the baby is premature. Studies show that THC is passed on through the breast milk (but not in amounts that would produce a high) There have been some reports of decreased feeding, weight loss and possible slowed mental development (although this has not been confirmed in controlled studies).

Not really known: There is little research on this important topic; of the little that has been done, some suggest it might impair a baby's motor development, reduce ability to nurse & decrease milk supply but none of these are fully proven. The answer likely depends on how much & how often. The safest course would be to avoid entirely but occasional use may be OK, esp. if there is a medical need. See comment:

Not well understood: Cannabis will most likely pass onto your baby. There isn't much direct evidence of risks, but cannabis does affect brain development and cognition in adolescents. I do not recommend nor advise that patients smoke, ingest or consume cannabis while pregnant or breast feeding.

Why risk the harm?: Not enough is known as this would not be an ethical study to expose infants to a potentially harmful drug. I thank you for asking, and hope if you are the one considering using marijuana, that you visit with your doctor for an alternative "treatment" that is better known / more likley to be safe for you both.