Whilst essential oils can be used during pregnancy albeit in a more diluted form and advised to not be ingested or used internally, some should be restricted or completely avoided on all counts, due to the high risk they pose to the health and development of the baby.
So why are some essential oils harmful?
It's all to do with the oils' chemical makeup. As with any substance, an essential oil is made up of many different, naturally combined constituents. Each constituent has a known effect on the body. The amount at which a particular constituent is present denotes the oils' overall therapeutic benefits, or on the flip side, its toxicity. This then determines how it should be best and most safely used.
With regard to pregnancy, toxicity relates to the constituents of an essential oil that are known to interfere with hormones or those that disrupt reproductive processes and the various stages of fetal development when they cross the placenta. With breastfeeding the concerns are associated with harmful constituents passing through mum and being ingested by baby. Research into essential oils and pregnancy is however, limited and much of the information that is available is often conflicting.
Robert Tisserand, the leading expert in essential oil safety, advises that the following essential oils should be restricted or completely avoided in pregnancy and breastfeeding (Essential Oil Safety, 2nd Edition)
Whether more essential oils are added to the cautions list will depend on the availability of information through studies, over time.
As a note, absolutes, (which are not all contained on the list and are oils that have been extracted from the plant using alcohol or chemical based solvents) have a higher concentration than their steam distilled counterparts. Absolutes also contain residues of the solvent used, therefore the 'natural' level of the remaining content is compromised. In terms of safety, Robert Tisserand cites;
" the likely risk of solvent toxicity from the use of absolutes is negligible, especially considering that the parts per million in an absolute are further diluted in an essential oil blend or aromatherapy product".
In summary, absolutes are usually safe to use, however, if you are looking for natural based products opt for pure essential oils instead.