Why it matters: Reduces risk of neural tube defects (NTDs) by up to 70%. The MRC Vitamin Study (1991) showed a 72% reduction in NTD recurrence with 4 mg/day. USPSTF Grade A recommendation (2023).

Who needs the high dose (4 mg/day): Previous pregnancy affected by NTD · Personal or partner history of NTD · Diabetes (pre-gestational) · Antiepileptic drug use (valproate, carbamazepine, phenytoin) · Obesity (BMI >30 — standard dose may be inadequate) · Malabsorption disorders · Some MTHFR variants (discuss with provider)

Food sources: Leafy greens, legumes, fortified cereals — but dietary folate alone is insufficient for NTD prevention. A supplement is required.

Note: Methylfolate (L-5-methyltetrahydrofolate) is the active form found in some prenatal vitamins. It is an appropriate alternative, especially for women with MTHFR variants, though evidence for superiority over standard folic acid is not conclusive for most women.

Why it matters: Iodine is essential for fetal thyroid development and brain growth. Severe deficiency causes cretinism. Mild-to-moderate deficiency (common in the US) is associated with neurodevelopmental effects.

ACOG recommends that all pregnant and breastfeeding women take a prenatal supplement containing iodine. Check the label — many prenatal vitamins do not contain iodine or contain inadequate amounts.

Source: Iodized salt, dairy, seafood. Vegan and dairy-free diets may be at higher risk for iodine deficiency.

Caution: Excess iodine can cause thyroid dysfunction. Do not exceed 1,100 mcg/day. Seaweed supplements may contain highly variable and excessive amounts.

Who needs extra iron: Women with iron-deficiency anemia before or during pregnancy · Heavy menstrual periods · Vegetarian/vegan diet · Multiple gestation · Close birth spacing. Standard prenatal vitamins contain 27 mg iron — sufficient for most non-anemic women.

Absorption tips: Take with vitamin C (orange juice) to enhance absorption. Avoid taking with calcium supplements, dairy, or tea within 2 hours.

Side effects: Constipation and nausea are common. Carbonyl iron or ferrous bisglycinate formulations are often better tolerated. Stool color will darken — this is normal.

Why it matters: Vitamin D supports fetal bone development, immune function, and may reduce risk of preterm birth and preeclampsia, though evidence is still evolving. Deficiency is common (up to 40% of pregnant women).

ACOG position (CO 804): Universal supplementation above 600 IU is not recommended without measuring 25-OH Vitamin D levels. Women at risk (limited sun exposure, darker skin, obesity, vegetarian diet) should have levels checked.

Testing: 25-OH Vitamin D is the appropriate test. Most prenatal vitamins contain 400–1,000 IU — adequate for replete women.

Why it matters: DHA is critical for fetal brain and retinal development. The fetus cannot synthesize adequate DHA independently and depends on maternal supply, especially in the third trimester.

ACOG (CO 804) states that evidence supports 200 mg DHA/day. Some experts recommend higher doses (400–600 mg) for women who consume little fish.

Sources: Fatty cold-water fish (salmon, sardines, herring, mackerel) are the richest sources. Algae-based DHA supplements are an appropriate option for vegetarians/vegans and avoid mercury concerns.

Mercury note: Avoid high-mercury fish during pregnancy: shark, swordfish, king mackerel, tilefish, bigeye tuna. 2–3 servings/week of low-mercury fish (salmon, shrimp, tilapia) is safe and encouraged.