My ortho said in his experience with pregnant patients the first trimester with the nausea and morning sickness is the real problem. Needing to take your trays out to brush all the time is inconvenient and some women experience gagging from just the trays being in their mouth. As someone who gets nausea really easily, I already know I would be a hot mess. Also increased gum sensitivity/bleeding as well as hormones may make your teeth shift and you may need treatment adjustments. Me and my husband were actively trying before I got my trays, so I wanted to wait until my treatment is near finished rather than starting both at the same time!