Several common herbs and herbal supplements pose real risks during pregnancy, from triggering contractions to damaging the liver or affecting fetal development. A large multinational study classified 27 out of 126 herbal medicines as contraindicated in pregnancy, and many of the most dangerous ones are readily available over the counter. Here’s what you need to know about which herbs to avoid and why.
Herbs That Stimulate the Uterus
The largest category of dangerous herbs in pregnancy includes those classified as emmenagogues or uterine stimulants. These herbs can promote uterine contractions, potentially leading to miscarriage or preterm labor. The most well-documented ones include blue cohosh, black cohosh, shepherd’s purse, motherwort, and lovage.
Blue cohosh deserves special attention because it’s widely used by midwives as a labor-inducing aid, with roughly 64% of surveyed U.S. midwives reporting its use. But the published case reports are alarming: blue cohosh taken near delivery has been linked to perinatal stroke, heart failure and shock in newborns, and severe multi-organ injury from oxygen deprivation. Lab studies also suggest it may cause birth defects and be toxic to embryos. It has documented abortifacient properties as well.
Black cohosh carries similar concerns as a uterine stimulant, though it’s more commonly marketed for menopause symptoms. Motherwort and lovage, popular in some European and Eastern European traditions, are also contraindicated specifically because of their ability to promote uterine bleeding and contractions.
Pennyroyal: The Most Dangerous Herbal Oil
Pennyroyal is in a category of its own. This mint-family plant has been used historically as an insect repellent and abortifacient, but its oil is extremely toxic. Even a single tablespoon (15 mL) of pennyroyal oil can cause seizures, coma, cardiovascular collapse, acute liver failure, kidney damage, and death.
The toxic component is a compound called pulegone, which makes up about 80 to 85% of the oil. Your liver converts pulegone into a byproduct that directly destroys liver cells. Pregnant women are particularly vulnerable to this rapid-onset liver damage. Pennyroyal oil is still widely available and sometimes marketed as a general herbal tonic, which makes it especially dangerous for anyone who doesn’t know its risks.
Goldenseal and Berberine-Containing Plants
Goldenseal, European barberry, Oregon grape, and several other plants contain a compound called berberine. Pregnant and breastfeeding women should not use berberine in any form, and it should never be given to infants. The reason: berberine can cause or worsen jaundice in newborns and may lead to kernicterus, a type of brain damage caused by dangerously high levels of bilirubin. This isn’t a theoretical concern. The National Institutes of Health lists this as a direct warning.
Essential Oils That Cross the Placenta
Essential oil components are small, fat-soluble molecules that easily cross the placenta into fetal circulation. If an essential oil compound is present in your bloodstream, whether from inhaling it, applying it to your skin, or ingesting it, it will reach the fetus.
Two categories are considered the most dangerous. Wintergreen and sweet birch oils are composed almost entirely of methyl salicylate (96 to 99.5% and 90.4%, respectively). When absorbed through the skin, methyl salicylate converts to salicylic acid, which crosses the placenta and has been shown to cause restricted fetal growth and congenital abnormalities in animal studies. These oils should be avoided entirely during pregnancy, by any route.
Savin oil, from the juniper species Juniperus sabina, contains high levels of a compound called sabinyl acetate. This oil is directly embryotoxic, causes liver damage, and acts as an abortifacient. Because no safe dose has been established, it should be completely avoided, especially during the first trimester.
Sage, Parsley, and the Dose Problem
Many herbs that are perfectly safe as seasonings become risky at medicinal or supplemental doses. This distinction matters because herbal supplements concentrate active compounds far beyond what you’d get from cooking.
Sage is a good example. It’s considered safe in food amounts, and some sources note it may be acceptable after 37 weeks of gestation. But high doses of sage can promote miscarriage. Turmeric in food is generally safe, but concentrated turmeric supplements can interact with other medications and affect blood sugar and blood cell counts. Ginger is well-studied for relieving pregnancy nausea, but the safe range tops out at about 1 gram per day. Cloves, fennel seeds, and senna are considered safe in the small amounts typically used in food, but there isn’t enough data to confirm the safety of large or supplemental doses.
The rule of thumb: a pinch in your recipe is different from a capsule or extract. If you’re using an herb in supplement form, assume it’s a medicinal dose until proven otherwise.
Soy and Phytoestrogen Concerns
Soy products and other phytoestrogen-rich herbs are a more nuanced concern. Phytoestrogens are plant compounds that mimic estrogen in the body, and the most common sources are soy-based foods containing isoflavones like daidzein and genistein. These compounds do reach the fetus: studies measuring amniotic fluid found that phytoestrogen concentrations vary by fetal sex, with levels in pregnancies carrying female fetuses roughly double those in male pregnancies.
There are concerns that phytoestrogens could act as endocrine disruptors, potentially affecting male reproductive development, though the data isn’t conclusive. Normal dietary soy intake hasn’t been definitively linked to harm, but concentrated isoflavone supplements are a different story. Until more is known, high-dose phytoestrogen supplements are worth avoiding during pregnancy.
Herb-Drug Interactions During Pregnancy
Even herbs that aren’t directly harmful to the fetus can become dangerous by interacting with medications you’re already taking. A cross-sectional study found 34 potential interactions between commonly used herbal supplements and pregnancy medications. The herbs most frequently involved were aloe, chamomile, cranberry, fish oil, ginger, ginseng, grapefruit, and sage.
Most of these interactions were rated as moderate, but one stood out as potentially major: ginger combined with nifedipine, a blood pressure medication sometimes prescribed in pregnancy. The interactions work through two main pathways. About half involve the herbs amplifying or counteracting the drug’s effects directly. The other half involve the herbs interfering with liver enzymes that process medications, which can raise drug levels in your blood to unsafe concentrations. Cranberry, for example, can inhibit an enzyme responsible for breaking down certain medications, potentially causing those drugs to accumulate.
Fish oil, often taken alongside prenatal vitamins, can increase bleeding risk if you’re on blood thinners and may cause blood pressure to drop too low if combined with certain antihypertensive medications.
Cannabis and Other Commonly Used Herbs
Cannabis was specifically flagged among contraindicated herbs used by pregnant women in North America. Papaya, particularly unripe papaya, was also listed. Cowberry (lingonberry in supplement form) was the single most frequently used contraindicated herb in a multinational study, used by nearly 30% of the women taking contraindicated products, though its risks are less dramatic than some of the herbs listed above.
The broader issue is that many women don’t think of herbal products as medications. The American College of Obstetricians and Gynecologists recommends that all nutritional supplements and herbal products be reviewed before and during pregnancy, specifically because patients often don’t mention them during medical visits. If you’re taking any herbal product, whether it’s a tea, tincture, capsule, or essential oil, bring it up with your provider so they can check it against your full medication list.