A:Certain essentials oils are considered too powerful for use during pregnancy as they may stimulate the production of certain hormones and can encourage bleeding. Please consult your physician or a reputable aroma therapist before using any essential oil. Some essential oils to refrain from using throughout pregnancy include: Ajowan, Basil, Camphor, Cederwood, Chamomile, Clary Sage, Cypress, Fennel, Hyssop, Juniper Berry, Marjoram, Rose Geranium, Rosemary and Thyme to name a few.