Recommendation: All women of childbearing age should take an iodine supplement of 150 µg/d. Although some prenatal vitamins include iodine, the assumption should not be made that all do. Once a women becomes pregnant, recommended iodine intake increases to 220 to 250 µg/d. Providers may encourage iodized salt as a primary salt source, with instructions to the patient to check the labels carefully to ensure that the salt product is iodized.


Magnesium is a mineral essential to the human body. It is necessary for more than 300 enzymatic reactions and has an effect on basic physiology, including blood pressure, bone health, and cardiovascular health.16 Magnesium deficiency is associated with chronic fatigue, tetany, altered glucose metabolism, and electrolyte disturbances; during pregnancy, magnesium deficiency has been associated with preterm labor, pre-eclampsia, and maternal muscle cramps.17 The current recommended daily allowance (RDA) for magnesium in women is 310 to 320 mg/d; pregnant women are advised to consume 350 to 400 mg/d.16 The normal serum magnesium range is 0.65 mmol/L to 1.05 mmol/L; however, serum magnesium concentrations are poor predictors of intracellular/total body magnesium content.18

An estimated 68% of Americans do not consume the RDA of magnesium, whereas 19% fail to consume even half of the RDA of magnesium.19 Providers should consider the low levels of magnesium in the general population when advising women on preconception care. Providers can also rely on common clinical signs of hypomagnesemia, including loss of appetite, nausea, vomiting, fatigue, and weakness. Severe magnesium deficiency may present with symptoms including numbness, tingling, muscle contractions, cramps, seizures, and arrhythmias.18

Recommendation: Women of childbearing age may benefit from magnesium, 300 mg/d, supplementation. Magnesium received in the formulations of aspartate, citrate, or lactate appear to be better absorbed with improved bioavailability; magnesium oxide has demonstrated extremely low bioavailability.20 In addition to taking an oral supplement, magnesium status can also be improved with transdermal absorption via regular Epsom salt baths or foot soaks. Epsom salt is magnesium sulfate that is absorbed through the skin.

Vitamin D

Vitamin D is a fat-soluble vitamin that is produced endogenously with exposure of skin to ultraviolet radiation in sunshine. Naturally present in very few foods (primarily fish), vitamin D balances calcium levels and has protective roles in immune function and inflammation.21 An estimated 46% to 82% of US adults have vitamin D deficiency; the highest risk of deficiency is in dark-skinned individuals.22

Routine testing for vitamin D levels is not recommended. Therefore, indirect measures of vitamin D levels can be estimated via an individual’s exposure to sunlight, latitude, clothing coverage, and skin pigmentation.23

Vitamin D deficiency has been linked to multiple adverse perinatal outcomes in pregnancy, including pre-eclampsia, gestational diabetes, low birth weight, bacterial vaginosis, and preterm delivery.24 In the general population, deficiency of vitamin D is also correlated with multiple chronic diseases, including cardiovascular disease and cancer.22

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Recommendation: Vitamin D, 4000 IU/d, is recommended for all women of childbearing age and pregnant women.25 Effective bioavailable forms include both capsules and high-concentration drops.