Pregnancy Supplements: Take vs Avoid

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At its core the supplement question in pregnancy is simple: a short list of nutrients has good evidence and clear guideline backing, and most of the rest is either unnecessary or, in a few cases, actively risky. The complication is that the same shelves stocking prenatal vitamins also stock high-dose single nutrients and herbal blends that were never studied in pregnant women. What follows is the take list with doses and an honest grade of the evidence, then the avoid list — background for a conversation with your clinician, not a substitute for one.

Take: the evidence-backed core

Folic acid (or folate) — the clearest win. This is the single most important supplement in pregnancy, and the evidence is unusually strong. The randomised MRC Vitamin Study found that folic acid taken around conception cut the recurrence of neural tube defects by 72% in women who had already had an affected pregnancy [1], and the Hungarian trial by Czeizel and Dudás showed that a multivitamin containing 0.8 mg of folic acid prevented the first occurrence as well [2]. On this basis, ACOG and the U.S. Preventive Services Task Force recommend 400–800 µg daily for anyone who could become pregnant, started at least a month before conception — the neural tube closes by about week four, often before a pregnancy is recognised. Women with a prior affected pregnancy, diabetes, or on certain anti-seizure drugs are usually advised 4 mg/day under medical supervision. Grade: strong.

Iron — treat deficiency, screen everyone. Iron-deficiency anaemia is common in pregnancy and is linked to preterm birth and low birthweight. A Cochrane review of daily oral iron found it reduced maternal anaemia at term by roughly 70% and iron deficiency by more than half, though the effect on infant outcomes was less clear-cut [3]. The pregnancy RDA is 27 mg, the amount in most prenatals, while established anaemia is treated with higher doses guided by bloodwork. The practical answer is to check ferritin and haemoglobin rather than guess. Grade: strong for deficiency, moderate for routine use.

Iodine — for fetal brain development. Iodine is needed to make the thyroid hormone that drives fetal neurodevelopment, and the first trimester is the vulnerable window. A meta-analysis of individual data from three birth cohorts linked lower maternal iodine status with modestly lower verbal IQ in children, concentrated before 14 weeks' gestation [4]. The American Thyroid Association and WHO advise a total intake around 150–250 µg/day, and the ATA specifically recommends a prenatal containing about 150 µg. Many U.S. prenatals still contain no iodine, so the label is worth checking. Grade: moderate to strong.

Vitamin D — correct deficiency. A Cochrane review of 30 trials concluded that vitamin D supplementation in pregnancy probably reduces pre-eclampsia, gestational diabetes, and low birthweight, while making little or no difference to preterm birth [5]. The benefit is most credible as deficiency correction rather than a universal boost. Most prenatals supply 400–600 IU; many clinicians use 1,000–2,000 IU/day, and treat confirmed deficiency with higher doses against a measured 25(OH)D level. Grade: moderate.

DHA / omega-3 — a real preterm signal, not a guaranteed smarter baby. This is where marketing outruns the data. The 2018 Cochrane review of 70 trials found that omega-3 long-chain fatty acids reduced preterm birth before 37 weeks (13.4% to 11.9%) and early preterm birth before 34 weeks (4.6% to 2.7%), both high-certainty [6]. But the same review found very little effect on childhood cognition, IQ, vision, or language — mostly low-quality and largely null. So the honest case for DHA (commonly 200–300 mg/day, from algal or low-mercury fish oil) rests on prematurity, not on a measurably smarter child. Grade: moderate (preterm), weak (neurodevelopment).

Choline — promising but unsettled. Choline is genuinely important for fetal brain development, and most pregnant people fall short of the 450 mg/day adequate intake; it is also frequently absent or underdosed in prenatals. The catch is that human evidence for supplementing beyond an adequate diet is mixed: a 2025 systematic review of four randomised trials and five observational studies concluded that current data are insufficient to confirm a neurodevelopmental benefit [7]. Choline-rich foods (eggs, dairy, meat) are a reasonable first move, with supplementation a defensible add-on framed as emerging rather than established. Grade: emerging / uncertain.

A standard prenatal multivitamin. ACOG advises a daily prenatal as the practical vehicle for folic acid, iron, iodine, and vitamin D. A pregnancy-specific formulation matters precisely because it caps the fat-soluble vitamins at safe levels — the bridge to the avoid list.

Avoid or approach with caution

High-dose preformed vitamin A (retinol) — teratogenic. This is the most important "avoid": preformed vitamin A is a known human teratogen at high doses. In a prospective study of over 22,000 pregnancies, women taking more than 10,000 IU/day from supplements had roughly five times the rate of cranial-neural-crest birth defects, with an apparent threshold near 10,000 IU/day [8]. Standard prenatals keep retinol well below this, often delivering much of their vitamin A as beta-carotene, which is not teratogenic. The danger comes from separate high-dose vitamin A pills, cod-liver oil layered on top of a prenatal, and prescription retinoid acne drugs (isotretinoin), which must be stopped before conception. Keep preformed vitamin A below about 3,000 µg RAE (10,000 IU) per day.

Excess iodine, including kelp. Iodine illustrates that more is not better: both too little and too much disrupt the fetal thyroid. Kelp and seaweed supplements can deliver wildly variable, sometimes enormous doses, and chronic excess can cause fetal thyroid dysfunction. Stay near the 150–250 µg/day target from a prenatal rather than layering on kelp.

High doses of anything fat-soluble. The same logic extends to vitamins A, D, E, and K, which accumulate. Vitamin E is a good example: high-dose supplementation in pregnancy has not shown benefit and has been linked to signals of harm in some trials, so megadoses well above the 15 mg/day requirement are not advised.

Herbal and "natural" products. Most botanicals have never been tested for safety in pregnancy and are not regulated for content or purity, and some carry specific concerns: vitex (chasteberry) and dong quai act on reproductive hormones, while blue cohosh, pennyroyal, and herbs traditionally used to stimulate the uterus carry abortifacient or cardiotoxic risk. The default for any herbal supplement in pregnancy should be no unless a knowledgeable clinician has specifically cleared it.

Contamination and quality. Because supplements are loosely regulated, products have turned up adulterated with heavy metals, undeclared drugs, or unlabelled ingredients — not an abstract risk in pregnancy. Favour pregnancy-specific products with independent third-party testing, and be skeptical of imported herbal blends and "pregnancy detox" formulas.

The practical takeaway

Build the regimen around a pregnancy-specific prenatal supplying folic acid, iron, iodine, and vitamin D; confirm iron and vitamin D status with bloodwork rather than guessing; and add DHA mainly for its preterm-birth benefit, with choline as a reasonable emerging option. Then keep the fat-soluble vitamins capped, leave the kelp and herbal blends on the shelf, and run anything else past your obstetrician or midwife first. Short, boring, and evidence-based is exactly what you want here.

Sources

1. MRC Vitamin Study Research Group. "Prevention of neural tube defects: results of the Medical Research Council Vitamin Study." Lancet, 1991;338(8760):131–137. PMID 1677062.
2. Czeizel AE, Dudás I. "Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation." N Engl J Med, 1992;327(26):1832–1835. PMID 1307234.
3. Peña-Rosas JP, De-Regil LM, Garcia-Casal MN, Dowswell T. "Daily oral iron supplementation during pregnancy." Cochrane Database Syst Rev, 2015;(7):CD004736. PMID 26198451.
4. Levie D, Korevaar TIM, Bath SC, et al. "Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data." J Clin Endocrinol Metab, 2019;104(12):5957–5967. PMID 30920622.
5. Palacios C, Kostiuk LK, Peña-Rosas JP. "Vitamin D supplementation for women during pregnancy." Cochrane Database Syst Rev, 2019;(7):CD008873. PMID 31348529.
6. Middleton P, Gomersall JC, Gould JF, Shepherd E, Olsen SF, Makrides M. "Omega-3 fatty acid addition during pregnancy." Cochrane Database Syst Rev, 2018;11(11):CD003402. PMID 30480773.
7. Gould JF, Hines S, Best KP, Grzeskowiak LE, Jansen O, Green TJ. "Choline During Pregnancy and Child Neurodevelopment: A Systematic Review of Randomized Controlled Trials and Observational Studies." Nutrients, 2025;17(5):886. PMID 40077755.
8. Rothman KJ, Moore LL, Singer MR, Nguyen US, Mannino S, Milunsky A. "Teratogenicity of high vitamin A intake." N Engl J Med, 1995;333(21):1369–1373. PMID 7477116.