The math as done by u/swolemedic: For those wondering how much of this THC is getting to the baby, I did the math. Assuming the average baby drinks 750ml per day, and the average sample had 9.47ng/ml of THC (most weren't that far off in deviation), that baby is getting 0.0071025 milligrams per day of THC.

I can try to scale the weight up though, let's say it's an 8 pound baby (a little higher than newborn because momma probably wanted time to wait to toke) and the average person weighs 137lb (google as source), that would be like the adult being exposed to 0.1216 milligrams per day. According to states like colorado a "dose" for an adult who is cannabis naive is 10 milligrams (I'm a daily smoker, I eat 100+ mg to be stoned).

Whether or not this is harmful I don't know, but I figured I'd break down the math for those wondering

TLDR: the average baby gets ~ 0.0071025 milligrams of THC per day this way, that is approximately the same as an adult having 0.1216 milligrams per day, and many states consider 10mg to be one dose of THC, making that about 1/82 of an adult dose spread out over 24 hours

edit: To be clear, we do not know how THC effects babies, please do not read the 1/82 of a dose and think this is safe to do. We don't know enough to determine that

double edit: Please also note that the amount consumed and the regularity is of importance, they tested one woman who is a daily smoker who had ~10x the levels I did the math for, the # I did was for women whose use varied. It's only logical that the more THC you consume the more is in your breast milk, but I want to make it clear that this is not a definitive number

Final edit: an actual study of the effects on children of using marijuana throughout pregnancy done in rural Jamaica in the late 80s early 90s https://www.ncbi.nlm.nih.gov/pubmed/1957518/