Researchers say psychoactive compounds in marijuana easily cross the placenta [4], exposing the fetus to perhaps 10 percent of the THC — tetrahydrocannabinol — that the mother receives, and higher concentrations if the mom uses pot repeatedly.
Links
[1] https://www.npr.org/sections/health-shots/2018/01/29/580535951/is-smoking-pot-while-pregnant-safe-for-the-baby
[2] http://www.rethinkpot.org/tags/npr
[3] http://www.rethinkpot.org/tags/pregnancy
[4] https://www.acog.org/Clinical-Guidance-and-Publications/Committee-Opinions/Committee-on-Obstetric-Practice/Marijuana-Use-During-Pregnancy-and-Lactation