Several essential oils pose serious risks during pregnancy, from triggering uterine contractions to causing liver damage or harming fetal development. The most dangerous contain compounds called thujone, pulegone, or methyl salicylate, and they should be completely avoided. Even oils considered lower risk require caution: the general recommendation is to skip all essential oils during the first trimester and use only diluted, well-studied options after that.
The Most Dangerous Oils: Pennyroyal, Wormwood, and Tansy
A handful of essential oils are genuinely toxic during pregnancy, not just theoretically risky. These are the ones that have documented links to miscarriage, severe liver damage, or nervous system harm.
Pennyroyal tops the list. This oil contains high levels of a compound called pulegone, which is both an abortifacient (meaning it can cause miscarriage) and a liver toxin. Pennyroyal poisoning causes severe liver damage, internal hemorrhage, and fluid buildup in the lungs. It has historically been used as a folk remedy to induce abortion, which is precisely why it is so dangerous for anyone hoping to maintain a pregnancy.
Wormwood (the plant famously used in absinthe) is another oil to avoid entirely. It’s rich in thujone, a compound that can cross the blood-brain barrier and cause seizures. In reproductive studies, wormwood oil has been classified as both embryo-toxic and abortifacient. Related oils from the same plant family carry similar risks: mugwort, tansy, sea wormwood, and white wormwood all contain thujone and are considered neurotoxic.
Tansy oil is neurotoxic and carries a specific risk of convulsions. Thuja (from the cedar family) is both an abortifacient and a contraceptive. Ingesting thuja oil can cause seizures, dangerously low blood pressure, and severe gastrointestinal distress.
Oils That Contain Thujone
Thujone is the compound that makes many of these oils dangerous. It interferes with a key signaling system in the brain that normally prevents overexcitation, which is why it causes muscle spasms and convulsions. The following oils all contain significant amounts of thujone and should be avoided during pregnancy:
- Sage (Dalmatian sage specifically, not clary sage): classified as embryotoxic
- Wormwood: abortifacient and embryo-toxic
- Mugwort: neurotoxic
- Tansy: neurotoxic, risk of convulsions
- Thuja and Western red cedar: abortifacient and neurotoxic
- Genipi: neurotoxic
- Lanyana: neurotoxic
Note that common sage (the kind you might have in your spice rack) and clary sage are different plants with different chemical profiles. Common sage oil contains thujone and is the one to avoid. Clary sage has its own risks, covered below.
Oils That Contain Pulegone
Pulegone is a liver toxin and abortifacient found in a smaller group of oils. Beyond pennyroyal, these include:
- Buchu oil (both common varieties): abortifacient, with some types also causing liver damage
- Lesser calamint: abortifacient and liver-toxic
These oils are less commonly found in mainstream aromatherapy products, but they do appear in herbal remedy shops and some traditional medicine preparations. If you see them listed as ingredients, steer clear.
Wintergreen and Birch: The Salicylate Risk
Wintergreen and birch essential oils are almost entirely composed of methyl salicylate, which your body processes the same way it processes aspirin. Salicylates are well known to cause problems in pregnancy, particularly in the third trimester, when they can affect fetal blood circulation. These two oils are concentrated enough that even small amounts applied to skin deliver a meaningful dose. They appear on virtually every “avoid during pregnancy” list for this reason.
Oils That Stimulate Contractions
Some essential oils don’t contain outright toxins but may encourage your uterus to contract. Clary sage, lavender, and jasmine have all been studied for their ability to promote labor onset. The mechanism appears to involve oxytocin, the hormone that drives contractions. A combination of clary sage and lavender may increase oxytocin levels by reducing the stress hormones that normally suppress its release, and repeated use of clary sage, lavender, and jasmine together may gradually enhance oxytocin output.
This is exactly why some midwives use these oils near a woman’s due date. But earlier in pregnancy, stimulating uterine contractions is the last thing you want. Clary sage in particular is widely recommended to avoid until full term.
The Full Avoid List
Beyond the oils explained above, several others lack sufficient safety data or contain compounds with known risks. The complete list of essential oils typically flagged as unsafe during pregnancy includes: aniseed, sage, basil, wormwood, rue, mugwort, oak moss, tarragon, birch, hyssop, camphor, parsley, pennyroyal, tansy, thuja, and wintergreen. Camphor-containing oils are a particular concern because camphor itself is neurotoxic in concentrated form. Hyssop also contains compounds that can trigger seizures.
The list of oils to avoid is significantly larger than the list considered safe. Many oils land on the “avoid” list not because they’ve been proven harmful, but because they simply haven’t been tested enough to confirm they’re safe for a developing fetus.
Why Pregnancy Increases the Risk
Essential oils are made up of small, fat-soluble molecules. They pass readily through your skin, lungs, and digestive tract into your bloodstream. From there, they can reach the placenta, accumulate in placental tissue, and even cross the placental barrier to reach the fetus. This isn’t a theoretical concern; it’s how these molecules behave based on their chemistry.
Pregnancy also changes your skin. Hormonal shifts can make you more sensitive to irritation and more prone to allergic reactions from substances you previously tolerated. An oil that never bothered you before might cause redness, itching, or a full sensitization reaction when you’re pregnant.
How to Use Oils More Safely After the First Trimester
The first trimester is the most critical period for fetal development, and most aromatherapy guidelines recommend avoiding all essential oils during those first 12 weeks. After that, if you want to use oils from the “generally recognized as safe” category (like certain citrus oils or ginger for nausea), the International Federation of Professional Aromatherapists recommends keeping the dilution to 1% or less for any skin application. That translates to roughly 6 drops of essential oil per ounce of carrier oil. In a bath, use no more than 4 drops total.
Diffusing oils is generally considered lower risk than applying them to skin, since less of the compound enters your bloodstream. But even with diffusion, you’re still inhaling active molecules that cross into circulation through your lungs. Keep sessions short, ensure the room is well ventilated, and stick to oils that don’t appear on the avoid list.
Ingesting essential oils during pregnancy is never recommended. The concentrated compounds bypass any buffering your skin or lungs might provide and deliver a full dose directly to your digestive system and liver. Many of the most dangerous case reports, including pennyroyal poisonings, involve oral ingestion.