Scents are received almost instantly by the limbic system in our brain, which is considered the oldest, or most primitive, part of the brain. When you smell something familiar it provokes an immediate, definite, response. This is the case for all smells - synthetic, natural, good, bad etc. Essential oils are brought into the bloodstream and distributed throughout the body (including through the placenta and to the fetus) when inhaled, and are also absorbed through our skin if applied topically, having a more direct effect on the system of the body it is closest to. In all modes of application/use, they eventually make it to our liver, which filters out & eliminates what is no longer needed in the body. If you are considering aromatherapy for someone who is vulnerable (young children, the elderly, pregnant or breastfeeding mothers, those with a pre-existing medical condition) it would be wise to meet with a trusted practitioner, or if that isn't possible, research at reputable sources first, to ensure that the essential oils and mode of application you would like to use will be of maximum benefit, and not cause harm.
The sense of smell in pregnancy, and especially during labour, can be significantly heightened. Lavender suddenly smells like the oregano in your great-Aunt's spaghetti sauce, peppermint reminds you of your grandfather's muscle linament, and chocolate m&m's candy are pretty much dog breath. ;)
If you choose to use scent of any sort during labour (true essential oils for their healing value, or synthetics for emotional comfort only) I suggest putting a single drop on a cotton ball or tissue so that if it becomes offensive it can be flushed down the toilet and completely eliminated from your space. The last thing you want is to massage an oil all over your belly, only to have it make you nauseous, or cause an unwanted reaction, or cause your caregiver to have an allergic response, and not be able to easily get rid of it. If you have tried it on a cotton ball first, and everyone is comfortable with the smell, then it is probably safe to use the diluted essential oil topically if you choose. A 1% dilution is considered appropriate for pregnancy/birth.
Essential oils like those I carry in my birth doula kit & reflexology bag can have a very positive effect on your well-being in labour when used appropriately. Lavender is a purifying and relaxing oil. Peppermint is clarifying and opening. Sweet orange is uplifting and cleansing. Clary Sage is grounding. And when certain essential oils are artfully combined like in my custom labour blend, you get a synergistic effect, where the whole is greater than the sum of its parts. True essential oils are also antibacterial &/or antiviral &/or antifungal &/or antiseptic in nature - - to different degrees depending on the specific essential oil. They can help with issues such as nausea, fatigue, swelling, emotional distress, muscle tension, pain, and many more, when used appropriately.
If you are going to purchase essential oils for use in pregnancy or labour, I suggest speaking with someone who is very experienced in the art & science of aromatherapy (not just a direct sales rep or a store clerk), so they can help you choose a brand, aroma, and method of application that will best meet your needs. If you would like to know more about aromatherapy, and specifically its use in pregnancy, please feel free to visit the following links:
Heart of Herbs
The Barefoot Dragonfly
National Association for Holistic Aromatherapy
International Federation of Professional Aromatherapy
I also have many books available in my lending library which address aromatherapy & pregnancy:
~ Aromatherapy: A practical guide to essential oils & aromamassage by Jan Balkam
~ Natural Healing For the Pregnant Woman by Elizabeth Burch & Judith Sachs
~ 475 Herbal & Aromatherapy Recipes by Demetria Clark
~ Aromatherapy & Massage For Mother & Baby by Alison England
~ Aromatherapy for Dummies by Kathi Keville
~ The Natural Pregnancy Book by Aviva Jill Romm
~ Pregnancy The Natural Way by Zita West
~ The Complete Book of Essential Oils & Aromatherapy by Valerie Worwood