Supplements During Pregnancy: What's Safe and What to Avoid

NHS-Recommended Supplements in Pregnancy

The NHS recommends all pregnant women take:

Folic acid — 400mcg daily From before conception through week 12. Reduces risk of neural tube defects (spina bifida) by 70–75%. Women with a personal or family history of NTDs, taking antiepileptic medications, or with diabetes may need higher doses (5mg, prescribed).

Women with MTHFR genetic variants may benefit from methylfolate (5-MTHF) instead — discuss with your midwife.

Vitamin D — 10mcg (400 IU) daily Required throughout pregnancy for fetal bone development and immune programming. Some evidence suggests higher doses (1,000–2,000 IU) may further reduce risk of pre-eclampsia, gestational diabetes, and low birthweight — discuss with your midwife.

These two are the non-negotiables. Everything else is additional.

Well-Evidenced Additional Supplements

Omega-3 (DHA specifically)

DHA (docosahexaenoic acid) is essential for fetal brain and retinal development. The brain is 60% fat, and 25% of that is DHA.

Requirements increase significantly in the third trimester when fetal brain development accelerates. UK dietary intakes of DHA are below recommended levels in most pregnant women.

Evidence: A 2018 Cochrane review found omega-3 supplementation in pregnancy reduces risk of preterm birth by 11% and severe preterm birth by 42%.

The NHS advises eating 2 portions of oily fish weekly during pregnancy (but avoiding high-mercury fish: shark, swordfish, marlin). If not eating oily fish, algae-based DHA (plant-sourced, avoiding fish entirely) is appropriate for vegans.

Dose: 300–600mg DHA daily. Choose algae-based DHA if vegan.

Iron

Iron requirements approximately double during pregnancy (from 15mg to 27mg/day). Demands increase most in the second and third trimesters. Most UK antenatal care includes routine haemoglobin testing — supplementation is guided by results.

Don't supplement iron without testing — iron excess is harmful to the fetus.

Iodine

Iodine is critical for fetal thyroid development and neurological function. Requirements increase by 50–100% in pregnancy. Severe deficiency causes cretinism; mild deficiency impairs cognitive development.

UK average intake has fallen significantly. Vegans and those avoiding dairy are at highest risk.

Most prenatal vitamins contain 150–200mcg iodine. Check yours. If not, add 150mcg potassium iodide.

Magnesium

Low magnesium in pregnancy is associated with pre-eclampsia, muscle cramps, and increased risk of preterm labour. Deficiency is common.

Important: High-dose IV magnesium sulfate is used clinically to prevent seizures in pre-eclampsia. Oral supplementation for prevention is not the same, but maintaining adequate magnesium status is evidence-supported.

Dose: 300–350mg elemental magnesium (food + supplement). Choose glycinate for best tolerance.

What to Avoid in Pregnancy

Vitamin A as retinol: At doses above 3,300 IU (1,000mcg retinol) daily, retinol is teratogenic. Cod liver oil (often high in retinol) should be avoided. Choose vitamin A supplements as beta-carotene (which is self-limiting in conversion) rather than retinol.

High-dose vitamin D: While deficiency is harmful, very high doses (above 4,000 IU) without monitoring are not recommended in pregnancy.

Herbal supplements as a category: Most herbal products have not been safety-tested in pregnancy. Avoid unless explicitly confirmed safe: - Avoid: Black cohosh, blue cohosh, evening primrose oil, pennyroyal, tansy - Avoid: High-dose ginger (small amounts in food are fine) - Avoid: Ashwagandha (animal studies suggest uterotonic effects) - Avoid: High-dose valerian

Vitamin B6 at high doses: The standard B6 in a prenatal vitamin is fine. Megadosing (200mg+) has been associated with neonatal seizures.

Fat-soluble vitamins in general: A, D, E, K — can accumulate and in excess may be harmful. Use the amounts in a standard prenatal vitamin, not standalone megadoses.

The Prenatal Vitamin Approach

A quality prenatal vitamin should cover your bases without requiring multiple separate supplements. Look for: - Methylfolate (5-MTHF) or folic acid 400–800mcg - Vitamin D3 1,000–2,000 IU - Iron (only if deficient — some prenatal vitamins are iron-free) - Iodine 150–200mcg - DHA at least 200mg (many standard prenatals skip this — add separately) - B12 as methylcobalamin

Separate omega-3 (DHA) is still recommended as most prenatal vitamins contain insufficient DHA.

Always inform your midwife and GP about all supplements you're taking. This guide is educational — individual situations vary significantly.