Without THC, marijuana would be of interest to no one. I followed up through adulthood the pups I had given these drugs the day before they were delivered. The pups looked pretty normal in appearance and size when thy were born. It wasn't until puberty that it was evident that they were underweight. We started to find hormonal imbalances that decreased testes weights, which started just before puberty but persisted up into the adult period of these animals who had only been exposed through their mother on the last day of her pregnancy or her through her milk the first six days of the nursing period.
Milk is so very fatty that it's a storage depot fro unmetabolised cannabinoids.
It has been shown in monkeys, rats, sheep and mice that THC definitely gets through milk in largely unchanged form. When we treated the males with cannabinoids, some of them had significant problems in making the females pregnant. And if those animals did make a pregnancy, many of the pups died as fetuses before the pregnancy went full term. In pups that survived birth, there was a significant increase in the number who died before they could eat on their own. Once the THC-treated males impregnated the untreated females, the females had more difficulty maintaining the pregnancy and raising those pups to adulthood.