Steaming is not done during pregnancy with the exception of when you have reached full term. Steaming during any other stage of pregnancy can clear out the uterine lining and embryo, inducing a miscarriage.
Steaming is traditionally considered safe to help aid labor at the end of pregnancy, except in the case of placenta previa. You can begin steaming for labor prep once you are full term. Most OBs will usually say you are full term at 37 or 38 weeks.
Steaming has also been shown to be safe and effective at helping to induce labor, decrease tearing and the need for episiotomies, it helps to soften the tissues of the pelvic floor which better prepares the birther’s body for labor. Vaginal steaming is also wonderful for postpartum healing and has been used for centuries to do so.