CHICAGO (AP) — Pregnancy started out rough for Leslie Siu. Morning sickness and migraines had her reeling and barely able to function at a demanding New York marketing job, so like rising numbers of U.S. mothers-to-be, she turned to marijuana.

"l was finally able to get out from under my work desk," said Siu, who later started her own pot company and says her daughter, now 4, is thriving.

There's no proof that cannabis can relieve morning sickness, and mainstream medicine advises against use in pregnancy because of studies suggesting it might cause premature birth, low birthweight and infant b...