Studies in breastfeeding mothers have demonstrated that there is significant and documented risk to the infant based on human experience, or it is a medication that has a high risk of causing significant damage to an infant. The risk of using the drug in breastfeeding women clearly outweighs any possible benefit from breastfeeding. The drug is contraindicated in women who are breastfeeding an infant.
The three studies that Puccio mentioned form the basis of a literature review on cannabis and lactation that Hale cites as a suggestion of "severe long-term neurobehavioral consequences." Building on those shaky foundations, and using carefully qualified statements, he goes on to introduce a number of unnecessarily ominous conclusions.
The first article (Day et al) used to support this claim finds no effect of marijuana exposure on any of the studied outcomes. The second (Fried et al) found transient effects in the immediate newborn period and at 4 years. They also found subtle possible long-term effects, but were unable to find a causal link, noting that 'the only conclusive statement would be that, if there are long term consequences of prenatal exposure to marihuana, such effects are very subtle.' The 3rd article (Hurd et al) did find that 'there was a 0.08-cm and 14.53-g significant reduction of foot length and body weight.' This is statistical significance, not clinical significance. These articles do not support your claim.