Beyond the prenatal: vitamin D, folic acid and iron

Supplements can be one part of planning for a pregnancy, but they’re personal: which ones, and how much, depends on you. This post covers three that come up often, and what the guidelines say. I’ve written separately about choosing a prenatal supplement.

Folic acid

The Public Health Agency of Canada recommends a daily multivitamin with 400 mcg (0.4 mg) of folic acid for anyone who could become pregnant, ideally started at least 3 months before pregnancy and continued throughout pregnancy and while breastfeeding. It also notes that some people need a higher dose.1 The Society of Obstetricians and Gynaecologists of Canada’s guideline likewise recommends a multivitamin containing 0.4 mg of folic acid for at least 2 to 3 months before conception.2

You may see other forms of folate on labels. These recommendations are for folic acid, so if you’re thinking about a different form, it’s worth checking with your care team first.

Iron

Health Canada’s reference values for iron are 18 mg a day for adult women and 27 mg a day in pregnancy, with an upper limit of 45 mg a day.3 Many prenatals contain iron for that reason.

Whether everyone should be screened for low iron in pregnancy isn’t settled. In 2024 the US Preventive Services Task Force concluded the current evidence is insufficient to assess the balance of benefits and harms of screening for iron deficiency and iron deficiency anemia in pregnant people.4 A Cochrane review found that daily iron supplements in pregnancy probably reduce iron-deficiency anaemia at term, with probably little to no difference in preterm birth.5

Because too much iron isn’t good either, it’s sensible to know your levels and talk through your options with your care team before taking extra.

Vitamin D

Guidelines disagree here. The Endocrine Society’s 2024 guideline suggests against routine vitamin D blood testing in pregnancy for generally healthy people,6 and the American College of Obstetricians and Gynecologists says there is insufficient evidence to support screening all pregnant women for vitamin D deficiency.7 Studies on both sides help explain why. A meta-analysis of 22 studies found low vitamin D levels in early and middle pregnancy went with a higher chance of pre-eclampsia (odds ratio 1.58).8 A different meta-analysis found that vitamins in pregnancy were linked with less pre-eclampsia overall, though trials on their own were negative overall, and vitamin D alone lowered it in the trials.9 In Iran, a city with a vitamin D screening and treatment program had less pre-eclampsia, gestational diabetes and preterm birth than a city without one, though these were two different cities.10 And in Melbourne, a study of 1,550 women found low vitamin D went with gestational diabetes, but concluded that routine testing of everyone didn’t appear warranted.11 These are links, not proof that treating a low level prevents a problem.

In my practice, I offer a vitamin D test to everyone I see in pregnancy, because I find it useful to know your level and to work from it. That’s my own clinical judgement and it goes beyond the guidelines above. You can always say no, and whether testing or supplementing makes sense for you is a decision to make with your care team.

Where I come in

If you’re planning a pregnancy or already pregnant, I’m happy to go through your supplements with you, look at your labs if you’ve had them, and talk about questions to take to your doctor or midwife. I work alongside your care team, not in place of it.