Key takeaways
- In a Hungarian trial that ran from 1984 to 1992, none of 2,819 pregnancies in the folic acid group had a neural tube defect, compared with 6 of 2,863 in the comparison group2.
- The U.S. Preventive Services Task Force gives folic acid its top grade: 0.4 to 0.8 mg a day, starting at least a month before conception1.
- In trials the Task Force reviewed in 2024, iron pills improved blood counts, but preterm births ran 5.5% with iron and 6.0% without, a gap that could be chance3.
- Two 2024 trials in 22,000 women in their first pregnancy, in India and Tanzania, found that 500 mg of calcium a day prevented preeclampsia about as well as 1,500 mg5.
- A 2021 Cochrane review found that probiotics in pregnancy didn’t clearly prevent gestational diabetes, and in four trials they raised the risk of preeclampsia10.
In 1984, Hungarian researchers began a trial that still anchors prenatal advice. Women planning a pregnancy took either a multivitamin with 0.8 mg of folic acid or a comparison pill with only trace minerals and a little vitamin C. They started at least 28 days before conception2.
The trial ended in 1992. Among 2,819 pregnancies in the folic acid group, there were no neural tube defects, the serious birth defects of the brain and spine. The comparison group had 6 among 2,863 pregnancies2.
Six cases sounds small. By our math, it’s about 2 per 1,000 pregnancies, or roughly 1 in 480. Nationwide, it adds up: the U.S. Preventive Services Task Force (USPSTF) estimates that about 3,000 pregnancies a year are affected in this country1.
Folic acid is the clearest supplement success story in pregnancy. The rest of a prenatal vitamin is a mix. Some nutrients have strong trials, some help only certain people, and some rest mostly on hope.
This article rates each one by the trials, the guideline body and the dose tested.
Folic acid is the one prenatal nutrient with an A grade
The USPSTF is an independent panel of prevention experts. In its August 2023 recommendation, it gives folic acid an A, its highest grade1. The recommendation is a daily supplement with 0.4 to 0.8 mg (400 to 800 mcg) of folic acid, starting at least one month before conception and continuing through the first two to three months of pregnancy1.
The panel aims it at everyone who could become pregnant, not just people who are trying1. Nearly half of U.S. pregnancies are unplanned, and the neural tube forms within 28 days after conception, a window that can close before planning starts16.
For every 10,000 U.S. live births, CDC estimates about 3.9 with spina bifida, 2.5 with anencephaly and 1 with encephalocele, the Task Force reports1. By our math, that’s about 7 in 10,000 births.
Few people start in time. In national surveys between 1998 and 2016, just 20% to 40% of women who had lately been pregnant, or hoped to be, said they’d taken folic acid pills around conception1. If the pregnancy wasn’t intended, the share who had taken them dropped by a factor of 4 to 51.
Safety looked reassuring. At ordinary doses, the panel saw no sign of serious harm1. Studies of autism, covering 761,125 people, found no increased risk, and one trial found no difference in twin births between 4 mg and 0.4 mg1.
Still, more isn’t automatically better. NIH’s Office of Dietary Supplements (ODS) lists an upper limit of 1,000 mcg a day of supplemental folic acid for adults 19 to 50. In 2001 to 2004 survey data, 33.4% of pregnant women ages 20 to 40 topped it6.
When the standard dose isn’t the plan. The Task Force advice doesn’t cover people who’ve had a pregnancy affected by a neural tube defect, who have a family history or take certain seizure medicines, or who take drugs that block folate, such as methotrexate1. For a prior affected pregnancy, CDC and the American College of Obstetricians and Gynecologists (ACOG) recommend a much higher dose under medical care6. Talk with your clinician before conception if any of these apply.
MTHFR and methylfolate. ODS states that folic acid is the only form of folate currently proven to reduce neural tube defects6. The Task Force says more research is needed on how gene variants such as MTHFR should shape supplement choices1. Our article on supplement forms covers CDC’s answer to the MTHFR claim.
Iron fixes blood counts, but trials haven’t shown it changes births
The Task Force’s August 2024 verdict on routine iron, for both blood tests and pills in pregnant people without symptoms, was an I statement, short for insufficient evidence3. An I is a call for more research, not a thumbs-down.
The trial evidence shows why. Across 16 trials, iron raised hemoglobin and ferritin, the protein that reflects iron stores3. In four trials with 2,230 women, anemia from iron deficiency at term fell from 19.8% of women to 8.6%3.
But the outcomes families care about barely moved. In five trials with 16,827 women, preterm birth was 5.5% with iron and 6.0% without, a difference that could be chance3. By our math, that’s about 5 fewer preterm births per 1,000, if the effect is real. Low birth weight was 2.7% versus 2.9%, and cesarean rates were 42.8% versus 41.5%3.
Side effects were mostly short-lived stomach upset: 3.6% of women on iron versus 2.3% without, in one trial of 12,5133.
Iron deficiency rises as pregnancy goes on. In national data from 1999 to 2006, it affected 6.9% of women in the first trimester, 14.3% in the second and 28.4% in the third3.
Guideline groups still lean toward iron. ACOG and CDC both advise routine low-dose iron in pregnancy, and ACOG pairs it with anemia checks along the way3. Our iron article covers testing and the risk of getting too much.
Calcium: a smaller dose matched a bigger one in 22,000 women
Calcium’s pregnancy role is about preeclampsia, a serious pregnancy complication involving high blood pressure. The World Health Organization (WHO) recommends 1,500 to 2,000 mg a day, split into three doses, for pregnant people in populations that get little calcium from food5.
That’s a lot of pills, and the burden has slowed programs5. So a team led by Pratibha Dwarkanath, a researcher in Bangalore, India, ran two trials, one in India and one in Tanzania13. Each enrolled 11,000 women pregnant for the first time and compared 500 mg a day with 1,500 mg, and both appeared in the New England Journal of Medicine in January 20245.
These were noninferiority trials. They asked whether the lower dose was not meaningfully worse than the standard one. What they found5:
| Trial | Outcome | 500 mg | 1,500 mg | Lower dose noninferior? |
|---|---|---|---|---|
| India | Preeclampsia | 3.0% | 3.6% | Yes |
| India | Preterm birth | 11.4% | 12.8% | Yes |
| Tanzania | Preeclampsia | 3.0% | 2.7% | Yes |
| Tanzania | Preterm birth | 10.4% | 9.7% | No |
The authors wrote that the 500 mg dose means fewer pills and would be expected to cost programs less13.
Whether calcium helps you depends on your diet. ODS describes a 2022 analysis of 30 trials in which women who already got enough calcium saw no benefit6. A 2018 Cochrane review found that high-dose calcium cut preeclampsia only in women with low intake, usually under 900 mg a day, and rated the evidence low quality6.
If dairy or calcium-fortified foods are part of most of your days, these trials may not apply to you. Our label check covers ACOG’s advice for low-intake diets, and our calcium article covers food sources.
DHA: the dose may matter most if your levels are low
DHA is an omega-3 fat found in seafood and algae. Our fish oil article covers the big Cochrane review on omega-3s and preterm birth; the newer question is dose.
In the ADORE trial, researchers led by Susan Carlson at the University of Kansas Medical Center gave nearly 1,100 pregnant women either 1,000 mg of DHA a day or 200 mg in the second half of pregnancy7. The lower dose matches what many prenatals with DHA provide, according to trade press8.
Early preterm births, before 34 weeks, occurred in 1.7% of the high-dose group and 2.4% of the standard group7. By our math, that’s about 7 fewer early preterm births per 1,000.
The gap was bigger in women who started with low DHA levels: 2% versus 4.1%7. That’s about 21 fewer per 1,000, by our math. In women who started with high levels, the dose made no difference7.
The National Institutes of Health funded the trial, published in 20217. It used algal DHA capsules from DSM, a company that sells DHA, according to trade press8.
The authors suggested checking DHA levels in pregnancy so women with low levels could consider a higher dose7. That’s a proposal, not yet standard care. If you eat little seafood, ask your clinician whether a DHA test or a higher dose fits you. Low-mercury fish remains the food-first route.
Choline and vitamin D: popular, with thin trial support
Choline. The National Academies set an Adequate Intake of 450 mg a day in pregnancy, as listed by ODS6. Our label check covers how far short most people and most prenatals fall.
The case for pills is thin. ODS notes that no professional society or agency gives specific advice on choline supplements in pregnancy6. The best-known link, between low choline and neural tube defects, comes from a few case-control studies whose authors warned of possible recall bias6. Eggs, a choline food named in the Dietary Guidelines, are the reliable route11.
Vitamin d. Here, the evidence shrank when it was checked. A Cochrane review updated in July 2024 had included 30 studies in its previous version9. After assessing each study’s trustworthiness, the authors set 20 aside to await further checks and excluded one9.
What remained was small: eight studies of vitamin D alone, with 2,313 women. Results on preeclampsia, gestational diabetes and preterm birth came from one to three studies each and were very uncertain9. One study of 1,134 women suggested fewer cases of severe bleeding after birth9. Three studies of 371 babies hinted at fewer low birth weight babies, but the authors couldn’t rule out an increase9.
The review’s searches ran through December 2022, and the reviewers called for larger, high-quality trials9. Until then, extra vitamin D beyond a standard prenatal is unproven for these outcomes.
Low-dose aspirin is a medicine, and a conversation
Aspirin isn’t a nutrient, but it belongs in this scorecard because it carries a Task Force grade for pregnancy. In September 2021, the USPSTF gave a B grade to low-dose aspirin, 81 mg a day after 12 weeks, for pregnant people at high risk of preeclampsia4.
High risk means any one of these: a past pregnancy with preeclampsia, twins or more, chronic high blood pressure, type 1 or type 2 diabetes before pregnancy, kidney disease, or an autoimmune disease such as lupus4. People with more than one moderate risk factor may also benefit. Those include a first pregnancy, a BMI over 30, a mother or sister who had preeclampsia, age 35 or older, lower income and conceiving through IVF4.
The panel also lists Black people as at moderate risk, because of social factors, not biology4.
Across 16 studies with 14,093 people, aspirin lowered preeclampsia risk by 15%4. Preeclampsia affects about 4% of U.S. pregnancies, and at least 8% of those with one high-risk factor4. By our math, a 15% cut from 8% means about 12 fewer cases per 1,000 high-risk pregnancies.
Aspirin also cut preterm birth by 20% and perinatal death by 21% in pooled trials4. Bleeding risks, including placental abruption and bleeding after birth, didn’t rise significantly4.
Don’t start aspirin on your own. Ask your clinician early in pregnancy whether your risk factors make it worth considering4.
Probiotics and the rest of the label
Some prenatals now contain probiotics, and ODS cautions that many such added ingredients haven’t been well studied in pregnant women6.
The best evidence isn’t encouraging. A 2021 Cochrane review pooled seven placebo-controlled trials with 1,647 women10. In six trials, probiotics didn’t clearly lower gestational diabetes, and the evidence was low certainty10.
Preeclampsia was the surprise. In four trials with 955 women, probiotics were linked to an 85% higher risk, a finding the reviewers rated high certainty10. The summary gave no absolute rates, so we can’t say how many extra cases per 1,000 that means.
The authors urged caution with probiotics in pregnancy10. Our probiotics article covers the separate question of eczema in babies.
Two other label lines deserve a word. Iodine is one the Dietary Guidelines single out for pregnancy11; our iodine article gives the amount. With vitamin A, the form decides whether too much can harm a baby6. Our label check and a companion article on supplement and herbal risks in pregnancy cover that.
The prenatal nutrient scorecard
This scorecard is our summary of the trials and guidelines above, rated on our four-level scale. Bring it to a prenatal visit. It isn’t a dosing guide.
| Nutrient | Who recommends | Dose tested | Best evidence | Rating |
|---|---|---|---|---|
| Folic acid | USPSTF, A grade (2023); ACOG, AAFP, AAP | 0.8 mg in a multivitamin before conception | Hungarian trial: 0 vs. 6 neural tube defects12 | |
| Iron | ACOG and CDC: routine low dose; USPSTF: I (2024) | Varied across 16 trials | Better blood counts; birth outcomes unclear3 | for anemia; not enough evidence for births |
| Calcium | WHO, for low-intake populations | 500 vs. 1,500 mg a day | Low dose noninferior for preeclampsia in 22,000 women5 | , mainly for low-intake diets6 |
| DHA | Trial authors suggest testing levels | 1,000 vs. 200 mg a day | Fewer early preterm births, mostly with low DHA7 | |
| Choline | AI of 450 mg; no society advises pills | No trials described by ODS | Observational links only6 | |
| Vitamin D | No pregnancy guideline reviewed here advises extra | Various | Cochrane 2024: very uncertain9 | |
| Low-dose aspirin | USPSTF, B grade (2021), high risk only | 81 mg a day after 12 weeks | 15% fewer preeclampsia cases4 | for high-risk pregnancies |
| Probiotics | None | Various strains | No clear diabetes benefit; more preeclampsia10 |
If you lean toward food over pills, the trials mostly back you. Nutrients with weak pill evidence, like choline, come from everyday foods such as eggs611. The clear exceptions are folic acid, because the window closes before most people know they’re pregnant, and the targeted options, like aspirin, iron or calcium, that make sense when a clinician finds a specific need.
Questions readers ask
Do I need a prenatal if I eat well?
The Dietary Guidelines advise talking with your clinician about a daily prenatal vitamin in pregnancy11. Folic acid is the one nutrient where the Task Force backs a pill for everyone who could become pregnant1.
Is methylfolate better than folic acid?
Folic acid is the only form proven to prevent neural tube defects, according to ODS6. Our article on supplement forms covers CDC’s view on MTHFR variants.
Should I add a separate DHA capsule?
Maybe, if you eat little seafood or your levels are low, but that’s a decision to make with your clinician7. Fish is the food-first source.
I took a probiotic early in pregnancy. Should I worry?
The Cochrane finding is a reason for caution, not panic, and it describes risk across groups of women, not your own outcome10. Tell your clinician what you took, so it’s part of your record.
The bottom line
Folic acid before conception has the strongest record of any prenatal nutrient, backed by a randomized trial and the Task Force’s top grade. Low-dose aspirin, a medicine, has good evidence for people at high risk of preeclampsia. Iron improves blood counts, calcium helps where diets run low in it, and DHA looks most useful when levels are low.
Choline and vitamin D pills remain unproven, and probiotics may do harm. Start with food, take folic acid early, and bring the scorecard to your clinician to decide what else, if anything, fits your pregnancy.
Related reading
About this article
This article was researched using U.S. Preventive Services Task Force recommendations and evidence reviews, Cochrane reviews, randomized trials, an NIH news release, NIH Office of Dietary Supplements fact sheets, the 2025–2030 Dietary Guidelines and trade press, all listed below. It was reviewed in October 2026. It is general education, not medical, legal or financial advice..
Sources
- U.S. Preventive Services Task Force. “Folic Acid Supplementation to Prevent Neural Tube Defects: Preventive Medication.” Final recommendation, August 1, 2023. uspreventiveservicestaskforce.org
- Viswanathan M, et al. (RTI International–University of North Carolina Evidence-based Practice Center). “Folic Acid Supplementation: An Evidence Review for the U.S. Preventive Services Task Force.” AHRQ Evidence Synthesis No. 145, May 2016. uspreventiveservicestaskforce.org
- U.S. Preventive Services Task Force. “Screening and Supplementation for Iron Deficiency and Iron Deficiency Anemia During Pregnancy: US Preventive Services Task Force Recommendation Statement.” JAMA, August 20, 2024 (doi:10.1001/jama.2024.15196). uspreventiveservicestaskforce.org
- U.S. Preventive Services Task Force. “Aspirin Use to Prevent Preeclampsia and Related Morbidity and Mortality: Preventive Medication.” Final recommendation, September 28, 2021. uspreventiveservicestaskforce.org
- Dwarkanath P, Muhihi A, Sudfeld CR, et al. “Two Randomized Trials of Low-Dose Calcium Supplementation in Pregnancy.” New England Journal of Medicine, January 2024;390(2):143–153 (abstract via Harvard Kennedy School Center for International Development). hks.harvard.edu
- NIH Office of Dietary Supplements. “Dietary Supplements and Life Stages: Pregnancy: Fact Sheet for Health Professionals.” Accessed October 2026. ods.od.nih.gov
- National Institutes of Health. “NIH-funded study finds higher dose of DHA associated with lower early preterm birth rate.” News release, May 24, 2021 (on Carlson SE, et al., EClinicalMedicine, 2021). nih.gov
- Nutrition Insight (trade press). “High daily dose of DSM’s DHA during pregnancy linked to decreased risk of preterm birth.” July 12, 2021. nutritioninsight.com
- Palacios C, Kostiuk LL, Cuthbert A, Weeks J. “Vitamin D supplementation for women during pregnancy.” Cochrane Database of Systematic Reviews, 2024, Issue 7, CD008873.pub5. cochranelibrary.com
- Davidson SJ, Barrett HL, Price SA, Callaway LK, Dekker Nitert M. “Probiotics for preventing gestational diabetes.” Cochrane Database of Systematic Reviews, 2021, Issue 4, CD009951.pub3. cochranelibrary.com
- U.S. Department of Health and Human Services and U.S. Department of Agriculture. “Dietary Guidelines for Americans, 2025–2030.” January 2026. cdn.realfood.gov
- MotherToBaby, Organization of Teratology Information Specialists. Home page. Accessed October 2026. mothertobaby.org
- HealthDay. “Low-Dose Calcium Noninferior for Reducing Risk of Preeclampsia.” January 11, 2024. healthday.com