Prenatal Supplements: What the Evidence Says About Folic Acid, Iodine, Iron, DHA and What to Avoid

The short version

• Folic acid before and in early pregnancy: grade A. Daily folic acid cut neural tube defects (serious birth defects of the brain and spine) by about two thirds in a Cochrane review (De-Regil 2015), and the US Preventive Services Task Force gives it an A recommendation (USPSTF 2023).
• Omega-3 (DHA and EPA): grade A for fewer preterm births. In a Cochrane review, early preterm birth fell from 4.6% to 2.7% (Middleton 2018).
• Iron: grade B, for fewer mothers with iron-deficiency anemia at the end of pregnancy (Finkelstein 2024). Get a blood test first.
• Iodine, vitamin D and choline: grade C. Each has a clear reason to be in a prenatal vitamin, but the trials are small or uncertain (Harding 2017; Palacios 2024; Gould 2025).
• Calcium for pre-eclampsia: grade B, little to no difference in large trials, although the WHO still advises it where diets are low in calcium (Cluver 2025).
• Avoid preformed vitamin A above 10,000 IU a day, which was linked to birth defects (Rothman 1995), and herbs such as ashwagandha, goldenseal, black cohosh and feverfew.

Every amount below is a guideline amount. Take it as advised by your midwife or obstetric team, who know your blood results and history.

How much folic acid should you take, and when?

A Grade A. The USPSTF recommends that everyone who is planning a pregnancy or could become pregnant take a daily supplement with 400 to 800 mcg (0.4 to 0.8 mg) of folic acid, and it rates its certainty of benefit as high (USPSTF 2023). In a Cochrane review of 5 trials, daily folic acid (alone or with other vitamins) lowered the risk of neural tube defects (risk ratio 0.31; 6,708 births; high-quality evidence), and it also lowered the risk of a repeat defect in women who had a previous affected pregnancy (De-Regil 2015). Start before you conceive if you can, because the trials gave it around conception.

If you have had a pregnancy with a neural tube defect, or have other risk factors, your obstetric team may advise a higher amount. Upper limit for adults: 1,000 mcg of folic acid a day, because more can mask vitamin B12 deficiency; take more only as advised by your midwife or obstetric team (NIH ODS; check your stack).

Should you take iodine during pregnancy?

C Grade C. Your baby's brain and thyroid depend on your iodine, and your needs rise in pregnancy. NIH's Office of Dietary Supplements reports that the American Thyroid Association recommends 150 mcg of iodine a day for women planning a pregnancy, pregnant or breastfeeding, and that only 34 of 59 best-selling prenatal vitamins it describes contained iodine (NIH ODS). The trials are thin. A Cochrane review of 11 trials with data (more than 2,700 women) found fewer cases of overactive thyroid after birth with iodine (3 trials, 543 women) but no clear difference in preterm birth or underactive thyroid, all on low-quality evidence (Harding 2017). Check your label: in a 2025 lab test of 47 prenatal products, most of those listing iodine contained more than 20% less than claimed (Borgelt 2025). Upper limit for adults: 1,100 mcg of iodine a day, because more can cause an underactive or overactive thyroid, and very large amounts near the birth have caused short-lived low thyroid function in newborns (LactMed: Iodine); take more only as advised by your midwife or obstetric team (NIH ODS; check your stack).

Do you need vitamin D in pregnancy?

C Grade C. Probably the amount in a prenatal vitamin, but the benefits are uncertain. The 2024 Cochrane review set aside 20 earlier studies over trustworthiness concerns, leaving 10. In the 8 that compared vitamin D with placebo or no treatment (2,313 women), the effect of vitamin D on pre-eclampsia, gestational diabetes and preterm birth was very uncertain; it may lower severe bleeding after birth (1 study) and low birthweight, though an increase could not be ruled out (Palacios 2024). Our database grades vitamin D C for pre-eclampsia, which matches. Upper limit for adults: 4,000 IU (100 mcg) of vitamin D a day, because more can cause high blood calcium; take more only as advised by your midwife or obstetric team (NIH ODS; check your stack).

Should you take iron during pregnancy?

B Grade B. Iron lowers anemia, but ask for a blood test rather than guessing. A 2024 Cochrane review of 57 trials (48,971 women) found that daily iron probably cut iron-deficiency anemia at term from 18.4% to 5.0% (7 trials, 2,704 women; moderate certainty) and may cut anemia overall (low certainty); its effect on side effects was very uncertain (Finkelstein 2024). The USPSTF found too little evidence to judge routine screening or routine iron for pregnant people without symptoms, measured by health outcomes for mother and baby (USPSTF 2024). Iron in prenatal vitamins is usually fine; higher treatment amounts are for anemia found on a test. Upper limit for adults: 45 mg of iron a day; more upsets the stomach, and iron pills are deadly to young children, so keep them out of reach; take more only as advised by your midwife or obstetric team (NIH ODS; check your stack). On levothyroxine? Read can I take iron, calcium or magnesium with levothyroxine?.

Does DHA or fish oil help in pregnancy?

A Grade A for fewer preterm births. A Cochrane review of 70 trials (19,927 women) compared omega-3 fats (supplements or food) with none. Preterm birth before 37 weeks fell from 13.4% to 11.9% (26 trials, 10,304 women) and early preterm birth before 34 weeks fell from 4.6% to 2.7% (9 trials, 5,204 women), both on high-quality evidence. Pregnancies running past 42 weeks rose from 1.6% to 2.6% (Middleton 2018). Fish counts: the FDA advises 8 to 12 ounces a week of lower-mercury fish in pregnancy (FDA fish advice). Ask your midwife about the amount; on a blood thinner, read can I take fish oil with Eliquis?.

Should you take calcium to lower pre-eclampsia risk?

B Grade B: little to no difference in large trials. Pre-eclampsia is high blood pressure with organ strain in the second half of pregnancy. In the 2025 Cochrane review, rates of pre-eclampsia were about the same with calcium as with placebo (6 trials, 15,364 women; low certainty), and the large trials alone gave the same answer with high certainty. Low and high doses did not differ in 2 trials (22,000 women at high risk in low-calcium populations) (Cluver 2025). The WHO still recommends 1.5 to 2.0 g of elemental calcium a day for pregnant women in populations with low dietary calcium (WHO). If you eat little dairy or other calcium-rich food, ask your midwife. Upper limit for adults: 2,500 mg of calcium a day from food and supplements, because more can cause kidney stones and high blood calcium; take more only as advised by your midwife or obstetric team (NIH ODS; check your stack).

Does choline matter in pregnancy?

C Grade C. Choline is a nutrient found in eggs, meat and fish, and the recommended intake in pregnancy is 450 mg a day (NIH ODS). In a trial of 26 women in the third trimester, babies of mothers given 930 mg a day processed information faster than babies of mothers given 480 mg (Caudill 2018). A 2025 review of 4 trials and 5 observational studies found most results null and the evidence too weak to say whether more choline helps children's development (Gould 2025). Few prenatal products list choline: 12 of 47 in the 2025 lab test (Borgelt 2025). Upper limit for adults: 3,500 mg of choline a day, because more can cause low blood pressure and a fishy body odor (NIH ODS).

Is a prenatal multivitamin better than folic acid and iron alone?

A Grade A for fewer low-birthweight babies in lower-income countries. A Cochrane review included 20 trials (141,849 women). In the 19 trials, all in low- and middle-income countries, that compared multivitamin and mineral supplements containing iron and folic acid with iron, with or without folic acid, the multivitamins cut low birthweight (risk ratio 0.88; 18 trials, 68,801 participants; high-quality evidence) and probably cut babies small for their age, with no difference in deaths around birth (Keats 2019). If you eat no animal foods, make sure your prenatal has vitamin B12, which you will also need while breastfeeding.

Do ginger or vitamin B6 help morning sickness?

C Grade C. A Cochrane review of 41 trials (5,449 women) found ginger may help, but the evidence was limited and inconsistent, and there was only limited evidence for vitamin B6 and the prescription combination doxylamine-pyridoxine (Matthews 2015). Use B6 for nausea only as advised by your midwife or doctor, and not long term at high doses. Upper limit for adults: 100 mg of vitamin B6 a day, because more can cause nerve damage (NIH ODS; check your stack). Vomiting that stops you keeping fluids down needs a doctor (see the hyperemesis protocol).

What we'd skip

• Preformed vitamin A above the upper limit. In a study of 22,748 pregnant women, babies of women taking more than 10,000 IU a day of preformed vitamin A from supplements had 4.8 times the rate of certain birth defects, and about 1 baby in 57 had a defect attributed to the supplement (Rothman 1995). Upper limit for adults: 3,000 mcg (10,000 IU) of preformed vitamin A a day, because more can cause birth defects and liver damage (NIH ODS; check your stack). Beta-carotene does not count toward this limit.
• Herbs flagged by NCCIH. The US National Center for Complementary and Integrative Health says ashwagandha should be avoided in pregnancy (NCCIH), goldenseal should not be used because its berberine can harm newborns (NCCIH), black cohosh may not be safe (NCCIH) and feverfew may affect uterine contractions (NCCIH). For many other herbs, pregnancy safety has simply not been studied.
• Melatonin without advice. NCCIH notes a lack of research on its safety in pregnancy (NCCIH).

Safety and interactions

• Add up your doses. A prenatal vitamin plus separate vitamin D, iron, iodine or vitamin A can pass an upper limit. Check your stack.
• Labels can be wrong. In the 2025 lab test, most prenatal products listing iodine fell more than 20% short, though none exceeded US Pharmacopeia limits for lead, arsenic or cadmium (Borgelt 2025). Our guide to choosing a quality supplement explains third-party testing.
• Medicines. Iron and calcium bind thyroid medicine, and omega-3 may add to blood thinners (links above). No interaction studies were found for most herbs in pregnancy.
• Don't stop a prescribed medicine on your own; ask the prescriber or pharmacist.

When to see a doctor

Contact your midwife or obstetric team the same day for:

• vomiting that stops you keeping fluids down, or signs of dehydration;
• a severe headache, changes in your vision, or sudden swelling of your face or hands, which can be signs of pre-eclampsia;
• bleeding, fluid leaking, severe tummy pain, or fewer baby movements later in pregnancy;
• extreme tiredness, breathlessness or a racing heart, which can be signs of anemia.

Tell your team about every supplement you take, including herbs and teas.

FAQ

What supplements should I take while pregnant? Folic acid (400 to 800 mcg a day, ideally from before conception) has the strongest evidence (grade A). A prenatal vitamin with iodine and vitamin D is standard, iron is for low iron found on a blood test, and omega-3 lowered preterm birth in trials (grade A). Agree each one with your midwife or obstetric team.

Which supplements should I avoid during pregnancy? Preformed vitamin A above 10,000 IU a day, which was linked to birth defects, and herbs that NCCIH flags, such as ashwagandha, goldenseal, black cohosh and feverfew. Ask before taking any herb, because many have no pregnancy safety data.

When should I start taking folic acid? Before you conceive if you can. The USPSTF advises it for everyone planning a pregnancy or who could become pregnant, because the trials gave it around conception.

Does calcium prevent pre-eclampsia? In the latest Cochrane review, large trials found about the same rate of pre-eclampsia with calcium as with placebo. The WHO still advises calcium for pregnant women in populations with low calcium intake, so ask your midwife if your diet is low in it.

Can I take vitamin B6 for morning sickness? Only as advised by your midwife or doctor. A Cochrane review found limited evidence for vitamin B6, and long-term high doses can cause nerve damage.

Related pages

• Pregnancy and delivery health: the evidence by supplement
• Pregnancy and prenatal health protocol
• Pre-eclampsia protocol
• Morning sickness protocol
• Gestational diabetes
• Supplements while breastfeeding
• Folic acid, iodine, vitamin D, iron, omega-3, calcium and vitamin B6 in our database
• Check your stack against the upper limits
• How we grade evidence

Sources

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