1. Legal doesn't mean scientifically tested, especially when it comes to pregnancy, breastfeeding, and even fertility.
2. Cannabis smoke has over 150 compounds. The most psychoactive of the byproducts of smoked cannabis is delta-9-tetrahydrocannabinol, it is a lipophilic molecule, meaning it's stored in body fat for a long time. In your blood stream the concentrations are cleared quickly by your liver, in your brain they last about 2 hours. Trace levels of cannabis can last in the blood stream for 4 days. No one knows how quickly you can clear it from your body completely, but cannabis byproducts are detectable in fat for weeks to months after consumption.
3. Breast milk can contain consumed cannabis. Smoking tobacco, as well as cannabis, gets small amounts of the agent into breast milk. About 2-3% of smoked cigarettes gets into mom's milk.
4. Secondary smoke also gets detected in breast milk.
5. No one knows if there is a critical time period for the baby's brain to be most or least affected by exposure to cannabis.
6. Tobacco smoke has been associated with low birth weight, and even some association with preterm birth. Cannabis has never been shown to cause either of these pregnancy problems. Studies are small, not all products have been studied.
4. Secondary smoke inhalation has been shown to cause detectable amounts of cannabis in milk.