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Are pregnant women taking too many supplements?

Antenatal care providers should reconsider widespread prescriptions for pregnancy supplements, and  promote whole food diets in pregnancy instead, write Linda Gallo and Shelley Wilkinson in Insight, suggesting that too many vitamins and minerals can be harmful to the unborn baby.

Multivitamins, promoted for “pregnant and/or breastfeeding” women, and even for those trying to conceive, generally contain folic acid, iodine, iron and a shopping list of other nutrients.

In high income nations, multiple micronutrient (MMN) supplementation during pregnancy is common. In Australia, among those who self-report taking at least one dietary supplement throughout pregnancy, the most common product is an MMN supplement.

There are, however, only two essential nutrients recommended for pregnancy: folic acid and iodine.

It is recommended that women supplement with 400 µg per day of folic acid from the month before conception to three months after they become pregnant. Taking a folic acid supplement is well known to reduce the chance of neural tube defects in the baby.

Guidelines also recommend expectant mothers take an iodine supplement of 150 µg each day and while breastfeeding. Women with a pre-existing thyroid disorder should seek medical advice first. Iodine is required for thyroid hormone production, which is crucial for brain development.

Mild iodine deficiency can lead to subtle cognitive and neurological problems in the child, and the Australian population is considered to be mildly iodine deficient.

Other nutrient supplements, including vitamin B12, vitamin D, iron, calcium and omega-3 fatty acids, are recommended for women with a diagnosed deficiency, pre-eclampsia risk, or for those who avoid certain food groups. Taking an individual supplement is best to meet requirements for these nutrients.

What’s behind the high rate of MMN supplements in pregnancy?

We conducted a study to identify the factors associated with MMN supplement use in Australian pregnancies, examining self-reported eating habits and supplement use of more than 120 women at around 28 weeks of pregnancy.

All women were receiving antenatal care at the Mater Mothers’ Hospital in Brisbane, one of Australia’s largest maternity hospitals. They were recruited as part of a pilot study for the Queensland Family Cohort (QFC), which aims to track participants for 30 years.

We found that four out of five women were taking an MMN supplement during the second trimester. Those who received private obstetric care, had private health insurance, and had greater alignment with the recommended consumption of meat and vegetarian alternatives, were more likely to report MMN supplement use. These factors are all associated with higher income.

In high income countries, where food is plentiful and mandatory fortification programmes are likely, MMN supplementation is associated with an increased risk of gestational diabetes and offspring adiposity – possibly attributed to high doses of elemental iron.

MMN supplementation more than five times per week has been associated with an increased incidence of autism spectrum disorder in offspring. There is, however, the common belief that supplements are safe to take during pregnancy, even when the woman’s diet is nutritionally adequate.

High reliance on supplements to meet (and often exceed) nutrient intake guidelines

Our earlier study examining dietary intakes of mothers and their partners found that most expectant women are not meeting the recommended serves for each of the five food groups. Only 25% ate enough vegetables, 42% met daily fruit recommendations, and less than 1% met the recommended intake of grain foods.

Dairy and meat or vegetarian alternatives were met by only 11% and 16%, respectively, of pregnant women.

For folate, iron and iodine, most women met intake guidelines, but there was high reliance on supplements to achieve this. In fact, more than 5% exceeded the upper level (UL) of intake for folic acid and almost 50% surpassed the UL for iron. Exceeding the UL was associated with higher than usual blood levels of the respective nutrients.

Risks of exceeding recommended nutrient levels

In countries with mandatory fortification programmes, like Australia, exceeding the UL for folic acid is reported frequently among supplement users of child-bearing age.

Exceeding the UL during the peri-conceptional period may decrease birth length and negatively affect child cognitive development at four to five years of age.

While folic acid supplementation beyond the first trimester, at 400 µg per day, may yield positive effects on the child’s cognitive development, supplementation during the third trimester might be associated with an increased risk of childhood asthma. The effects of high doses are mostly unknown and should be avoided unless medically indicated.

The most popular multivitamin brand in Australia contains 60 mg of elemental iron, which, if taken daily, increases the risk of maternal haemoconcentration. This is associated with adverse pregnancy outcomes, including small for gestational age, stillbirth, preeclampsia, gestational diabetes, and low birth weight. If taking an iron supplement, weekly (80-300mg) supplementation is as effective as daily (30-60mg) for preventing iron deficiency anaemia, and has fewer side effects.

In our study, about 20% of expectant mothers had iodine intakes greater than 350 µg per day, which has been linked to lower neurodevelopmental scores in children.

An important ongoing Australian study will define the effects of reducing supplemental iodine intake during pregnancy, in people who meet the nutrient guidelines from diet alone, on child cognitive development.

Some drawbacks

In our study, we assessed diet and supplement intake at a single time point only, in the second trimester. However, the relative contribution of diet versus supplements to nutrient adequacy have rarely been evaluated otherwise.

The sample size was also small, which may limit generalisability. It also prevented subgroup analyses of nutrient adequacy and did not allow us to examine effects on pregnancy or birth outcomes.

Calculations for daily dose of supplemental nutrients assumed that supplements were taken as a full dose. Where no brand was recorded, a conservative dose estimate was used, which may have led to under-reporting of daily supplement intake.

Need to emphasise whole food diets and not overdose on vitamins

In this largely high socio-economic population, MMN supplementation was high and not consistent with recommendations. Pregnant women need to be advised not to overdose on vitamins. More is not always better.

Megadoses of vitamins and minerals can be harmful to the unborn baby. A classic example is that of vitamin A, which can be toxic to a baby in large doses. Vitamin B6, which is present in many MMN and mineral supplements, has been reported to cause peripheral neuropathy (although not specifically recorded in pregnancy).

In response, the Therapeutic Goods Administration has strengthened labelling requirements, so products containing daily doses over 10 mg of vitamin B6 must carry a warning about this serious condition.

Our study confirms the need to promote whole food diets in pregnancy. An increase in vegetables and red meat or vegetarian alternatives for folate and iron are especially warranted.

Supplementation with folic acid during the peri-conceptional period should be continued at current dosage recommendations, along with other nutrient prescriptions when indicated. Supplementation with iodine throughout pregnancy may require consideration of dietary iodine sources to minimise risks associated with high intake.

Finally, antenatal care providers should reconsider widespread prescriptions for MMN supplements. These may be appropriate for some women following an assessment of their dietary patterns. Modifiable behaviours during the first 1 000 days of life influence developmental trajectories of adult chronic diseases, and this includes nutrient exposures during foetal development.

Dr Linda Gallo is a physiologist and lecturer in the School of Health at the University of the Sunshine Coast, Australia, with research interests in nutrition and cardiometabolic disease.
Associate Professor Shelley Wilkinson is an Advanced Accredited Practising Dietician and is a Project Officer in the Department of Obstetric Medicine at the Mater Mothers’ Hospitals, and Director and Principal Dietitian at Lifestyle Maternity, Australia.

Study details

Dietary supplements, guideline alignment and biochemical nutrient status in pregnancy: Findings from the Queensland Family Cohort pilot study

Linda Gallo, Sarah Steane,  Shelley Wilkinson, et al.

Published in Maternal & Child Nutrition on 10 November 2023


In high-income nations, multiple micronutrient (MMN) supplementation during pregnancy is a common practice. We aimed to describe maternal characteristics associated with supplement use and daily dose of supplemental nutrients consumed in pregnancy, and whether guideline alignment and nutrient status are related to supplement use. The Queensland Family Cohort is a prospective, Australian observational longitudinal study. Maternal characteristics, nutrient intake from food and supplements, and biochemical nutrient status were assessed in the second trimester (n = 127). Supplement use was reported by 89% of participants, of whom 91% reported taking an MMN supplement. Participants who received private obstetric care, had private health insurance and had greater alignment to meat/vegetarian alternatives recommendations were more likely to report MMN supplement use. Private obstetric care and general practitioner shared care were associated with higher daily dose of supplemental nutrients consumed compared with midwifery group practice. There was high reliance on supplements to meet nutrient reference values for folate, iodine and iron, but only plasma folate concentrations were higher in MMN supplement versus non-supplement users. Exceeding the upper level of intake for folic acid and iron was more likely among combined MMN and individual supplement/s users, and associated with higher plasma concentrations of the respective nutrients. Given the low alignment with food group recommendations and potential risks associated with high MMN supplement use, whole food diets should be emphasised. This study confirms the need to define effective strategies for optimizing nutrient intake in pregnancy, especially among those most vulnerable where MMN supplement use may be appropriate


Maternal and Child Nutrition article – Dietary supplements, guideline alignment and biochemical nutrient status in pregnancy: Findings from the Queensland Family Cohort pilot study (Open access)


InSight article – Pregnancy supplements – is there too much of a good thing? (Open access)


See more from MedicalBrief archives:


Omega-3 during pregnancy reduces risk of premature birth


Inexpensive intervention reduces birth complications – Botswana study


High iron deficiency prevalence yet 40% of pregnant women don’t get simple screening test


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