The vitamins most important to avoid or limit during pregnancy are preformed vitamin A (retinol), high-dose vitamin E, and excess iodine. Each carries specific risks to fetal development when taken above safe thresholds. Beyond individual vitamins, certain herbal supplements and even standard multivitamins (as opposed to prenatal formulas) can cause problems. Here’s what you need to know about each one.
Preformed Vitamin A (Retinol)
This is the single most important vitamin to watch during pregnancy. Preformed vitamin A, also called retinol, is the animal-derived form found in supplements, liver, and some fortified foods. It’s different from beta-carotene, the plant-based form in carrots and sweet potatoes, which your body converts to vitamin A only as needed and does not pose the same risk.
Doses of retinol exceeding 10,000 IU per day during the first trimester can cause birth defects affecting the brain, spinal cord, heart, and urinary tract. At doses above 25,000 IU per day, studies have found an increased risk of urinary tract malformations specifically. The WHO recommends a maximum of 10,000 IU daily during the first 60 days of pregnancy, when fetal organs are forming most rapidly. After that point, guidelines suggest no more than 25,000 IU per week.
If a pregnant woman’s retinol intake exceeds 10,000 IU per day, the American Heart Association recommends a fetal heart ultrasound because of a 1% to 2% risk of heart muscle problems in the baby. The practical takeaway: check your supplement labels for “retinol,” “retinyl palmitate,” or “retinyl acetate.” These are the forms that accumulate and cause harm. Beta-carotene listed on a label is not the same concern.
Liver and Food Sources
Liver is extraordinarily high in preformed vitamin A, and the amount varies widely depending on the animal and region. The Finnish Food Safety Authority and the UK’s National Health Service both recommend avoiding liver entirely during pregnancy. If you do eat it, portion size and frequency matter enormously, and checking a local food composition database is worth the effort because vitamin A content in liver can differ by a factor of ten depending on the source.
High-Dose Vitamin E
Vitamin E in standard prenatal amounts is not a concern, but high-dose supplements (typically 400 IU per day, the dose used in most clinical trials) have been linked to a 77% increased risk of your water breaking early at full term. This is called premature rupture of membranes, and while it sounds manageable, it can lead to infection, emergency induction, or complications during delivery. The same research, reviewed by the WHO, also found increased abdominal pain in women taking high-dose vitamin E.
On the positive side, vitamin E supplementation did reduce the risk of placental abruption (where the placenta separates from the uterine wall too early). But the WHO concluded there was no overall benefit for preventing preeclampsia, preterm birth, or fetal death. The balance of evidence doesn’t support taking vitamin E supplements beyond what’s in your prenatal vitamin.
Excess Iodine
Your body needs more iodine during pregnancy (about 250 micrograms per day), but going above that threshold creates real problems. The immature fetal thyroid gland can’t regulate how much iodine it absorbs the way an adult’s can. When too much iodine floods the system, the baby’s thyroid essentially shuts down, leading to fetal hypothyroidism and sometimes a goiter large enough to be visible on ultrasound.
A study of over 9,200 first-trimester women found that urinary iodine levels above 250 micrograms per liter significantly increased the risk of subclinical hypothyroidism, and levels above 500 micrograms per liter raised the risk of more serious thyroid hormone disruption. The risk here usually comes from stacking sources: a prenatal vitamin with iodine, plus iodized salt, plus seaweed or kelp supplements. Kelp tablets in particular can contain wildly inconsistent amounts of iodine and are worth avoiding entirely.
Vitamin D: Know the Upper Limit
Vitamin D is essential during pregnancy for bone development and immune function, and many women are deficient. But the tolerable upper intake level is 4,000 IU per day, combining both food and supplement sources. Studies have tested doses of 1,000, 2,000, and 4,000 IU in pregnant women without major safety concerns, but going above 4,000 IU without medical supervision risks calcium buildup in the blood, which can harm the kidneys and developing fetus. If your provider has tested your levels and prescribed a higher dose, that’s a different situation from self-supplementing.
Standard Multivitamins vs. Prenatal Vitamins
A regular multivitamin is not a safe substitute for a prenatal vitamin. The two are formulated differently in ways that matter. Prenatal vitamins contain higher amounts of folic acid and iron, which you need significantly more of during pregnancy. Standard multivitamins often contain preformed vitamin A at levels that could push you past safe limits, especially if combined with dietary sources. The Mayo Clinic specifically warns against taking extra multivitamins on top of a prenatal, noting that high doses of certain vitamins, particularly vitamin A, can harm the baby.
If you’re switching from a regular multivitamin to a prenatal, stop taking the regular one rather than doubling up. More is not better here.
Herbal Supplements to Avoid
Several herbal products are actively dangerous during pregnancy, and the lack of regulation makes dosing unpredictable. Two of the most well-documented are blue cohosh and black cohosh. Blue cohosh has been linked to neonatal congestive heart failure and perinatal stroke in case reports published in the New England Journal of Medicine and the Journal of Pediatrics. Black cohosh has been associated with severe sodium depletion during prolonged labor. Both are sometimes marketed as natural labor inducers, which makes them especially risky because women may take them in late pregnancy without realizing the danger.
Other herbs commonly flagged as unsafe during pregnancy include dong quai, pennyroyal, mugwort, and high-dose chamomile. The challenge with herbal supplements is that they aren’t held to the same manufacturing standards as pharmaceuticals, so the actual content of a capsule can vary dramatically from what’s on the label.
Timing Supplements to Avoid Absorption Problems
Even safe supplements can interfere with each other if taken at the wrong time. Calcium is one of the biggest offenders: calcium carbonate and magnesium oxide both significantly inhibit iron absorption when taken together. Since iron needs increase substantially during pregnancy, this interaction matters. Iron absorption is also reduced by the tannins in tea, the polyphenols in coffee, and the phytates in whole-grain cereals.
On the flip side, high doses of iron block zinc absorption when both are taken on an empty stomach. Since zinc is critical for fetal growth, this creates a real tradeoff. The practical solution is to take your iron supplement between meals with water or juice (not milk, tea, or coffee), and to separate calcium-rich foods or supplements from your iron dose by at least two hours. A modest iron dose absorbs better than a large one, both because your gut can handle it more efficiently and because it causes less interference with other nutrients.