Supplements to Avoid During Pregnancy (And What to Take Instead)

Pregnancy is a time when nutrition matters enormously — and when the wrong supplement can cause serious harm. The challenge is that supplement labels rarely indicate safety during pregnancy, and much online advice conflates standard prenatal vitamins with herbal or high-dose supplements that carry real risks.

Here's a clear breakdown of what to avoid, what's safe, and how to evaluate any supplement you're considering.

High-dose vitamin A: a clear teratogen

Vitamin A is essential for fetal development, but excess preformed vitamin A (retinol) is a well-established teratogen — it causes birth defects.

The tolerable upper limit during pregnancy is 3,000 mcg RAE (10,000 IU) per day of preformed vitamin A. Studies show that intakes above this threshold are associated with increased rates of cranial neural crest defects, heart malformations, and limb abnormalities. The risk is highest during the first trimester, when organogenesis occurs.

This is why most prenatal vitamins now provide vitamin A as beta-carotene — a provitamin the body converts to vitamin A only as needed, without accumulation risk. If your prenatal contains retinyl palmitate or retinyl acetate, check the dose carefully and avoid any additional vitamin A-containing supplements.

Herbs to avoid entirely during pregnancy

Several widely sold herbs have documented uterotonic or abortifacient effects:

• Blue cohosh (Caulophyllum thalictroides): contains alkaloids that stimulate uterine contractions and have caused neonatal heart failure in case reports.
• Pennyroyal (Mentha pulegium): contains pulegone, a compound that induces uterine contractions and is hepatotoxic. Has caused maternal deaths when used to attempt abortion.
• Wormwood (Artemisia absinthium): contains thujone, which is neurotoxic and may stimulate uterine contractions.
• Tansy: uterotonic, historically used as an abortifacient.
• Dong quai: estrogenic activity and possible uterine stimulation make it inappropriate during pregnancy.
• High-dose ginger (above 1,500 mg/day): while low-dose ginger (250 mg) has solid evidence for pregnancy nausea, high doses have theoretical uterotonic effects.

Supplements unsafe at high doses

• Vitamin E above 1,000 mg/day: associated with increased risk of congenital heart defects in observational data. Standard prenatal doses (15–30 mg) are safe.
• Selenium above 400 mcg/day: can cause toxicity. Many prenatal vitamins contain 50–70 mcg, which is appropriate.
• Iodine above 1,100 mcg/day: excess iodine can impair fetal thyroid function. The recommended intake during pregnancy is 220 mcg/day.

What is safe and evidence-supported

These are the nutrients with strong evidence for benefit during pregnancy:

Folate/methylfolate: 400–800 mcg/day reduces neural tube defect risk by 50–70% when taken before conception and in early pregnancy. Women with MTHFR variants benefit from L-methylfolate rather than synthetic folic acid.

Iron: 27 mg/day is the recommended intake during pregnancy. Iron deficiency anemia is the most common nutritional deficiency in pregnant women worldwide and is associated with preterm birth and low birth weight.

DHA (omega-3): 200–300 mg/day supports fetal brain and retinal development. The FDA has recognized omega-3 DHA as beneficial in pregnancy. Algae-based DHA avoids concerns about mercury in fish.

Vitamin D: deficiency is common and associated with adverse outcomes including preeclampsia and gestational diabetes. Most practitioners recommend 1,000–2,000 IU/day on top of the 400 IU in most prenatals, based on blood 25(OH)D levels.

Iodine: 150–220 mcg/day is needed for fetal thyroid hormone production and neurological development. Many women, particularly those who don't consume iodized salt or dairy, are deficient.

Choline: 450 mg/day is the adequate intake in pregnancy. Choline supports fetal brain development and is chronically underconsumed. Many prenatal vitamins contain inadequate choline; supplementing separately may be warranted.

How to evaluate any supplement during pregnancy

When considering any supplement beyond a quality prenatal:

1. Check the category: Is it a micronutrient with a defined safe upper limit, or an herbal product with unknown safety data?
2. Look for human pregnancy safety data, not just animal studies or traditional use.
3. Consult your OB or midwife before adding anything. Many supplements that are fine outside of pregnancy require more caution during it.
4. Default to less: if in doubt, a high-quality prenatal vitamin covering key bases is almost always the right starting point.

The bottom line

During pregnancy, the bar for supplement use should be higher, not lower. Avoid high-dose vitamin A, known uterotonic herbs, and anything with inadequate safety data. Focus your efforts on the nutrients with robust evidence: folate, iron, DHA, vitamin D, iodine, and choline.

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Managing your supplement routine during pregnancy takes careful oversight. Use Optimize free.

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• Supplements During Pregnancy: Beyond Just a Prenatal Vitamin