Women who take daily prenatal iron supplements may reduce the risk of anemia and low birth weight, a review concludes.

Iron deficiency is the most widespread nutritional deficiency globally, according to the World Health Organization. The agency has long recommended iron supplements in developing countries and prenatal nutrition guidelines in Canada, U.S. and UK do as well.

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The problem of serious iron deficiency tends to affect low-income countries. (Chaiwat Subprasom/Reuters)

To see how taking prenatal iron affects babies, researchers from U.S. and UK analyzed the results of more than 90 randomized trials and studies on prenatal iron use involving nearly two million women.

"Our findings suggest that the use of iron in women during pregnancy may be used as a preventive strategy" to improve maternal blood status and birth weight, Batool Haider of the departments of epidemiology and nutrition at Harvard School of Public Health in Boston and her co-authors concluded in Friday's issue of BMJ.

For example, for every 10 gram per litre increase in average hemoglobin concentration in the third trimester or at delivery, birth weight increased by 143 grams.

How long the iron was taken didn't make a difference after considering dose.

The researchers suggested exploring other feasible strategies for giving iron, such as comparing fortification and broadening dietary sources.

In general, Health Canada recommends taking a daily multivitamin that has 16 to 20 milligrams of iron to help have a healthy pregnancy.

The problem of serious iron deficiency tends to affect low-income countries, where some women may already have poor health status before pregnancy and may not be able to afford iron supplements, the UK's Royal College of Midwives noted in a comment on the research.

Using iron to increase birth weight could boost survival of newborns in the low range, the researchers said. They pointed to animal studies suggesting that the placenta may regulate transfer of iron and proteins.

Earlier this month, researchers in Australia said that taking iron tablets once a day offered no benefits in birth weight or better infant growth over taking the supplements just twice a week, based on a randomized control trial in rural Vietnam. That study was published in PLOS Medicine.