Despite the ever-growing body of research on essential oils, there is only a wee amount of study conducted on essential oil use during pregnancy- this owing to the unethical nature of conducting research on pregnant women.
The quick answer, though, is yes, many essential oils do cross the placenta (and cell membranes, and the blood brain barrier), which is why it is especially important to be mindful of essential oil concentration and purity during pregnancy. (Tillett & Ames, 2010)
Stephanie McBride is a published writer and certified clinical herbalist who has worked with herbal medicine for over 20 years. She has been in practice as a functional nutritionist and essential oil educator for Daily Nectar since 2009.