Why should certain oils be avoided during pregnancy?

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 aromatherapist 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.