You should note as well that essential oils can be dangerous if used excessively. Most aren't supposed to be used beyond minute amounts as in a drop or two. Some are also considered dangerous to be used during pregnancy. Always be very very careful with essential oils and check out any what's being said about ones you plan to use BEFORE you do. Better safe than sorry.
Here's a few useful links:
http://eethomp.com/AT/dangerous_oils.html
http://www.naha.org/faq_pregnancy.htm#13