I believe that's a "U. S. specialty" like the FDA quote on herbal infusions ("This product is not meant to treat ... la-diduh).
Essential oils (like many other plant-derived medications) can be very powerful so you need to know what you're doing. Depending on what you're using you could e. g. cause preterm contractions.
There are a couple of books out there. I like this one written by a German midwife: Ingeborg Stadelmann, Bewährte Aromamischungen - the English translation is Original IS Aroma Blends (the "IS" stands for the author's initials, in case you were wondering).