Iron was essential for healthy brain development, and a deficiency in babies and toddlers, when the brain was developing rapidly, was associated with small, but important differences in brain function, behavior and learning, according to the study from the University of Auckland.
It found that 7 percent of New Zealand newborns were iron deficient and established that iron stores were lower in babies whose mothers consumed higher quantities of milk during pregnancy.
"While milk is an important source of calcium it is a poor source of iron. Milk is also quite filling and so can reduce the appetite for other foods that are better sources of iron," pediatrician Associate Professor Cameron Grant said in a statement.
"Pregnant women with iron deficiency are more likely to go into premature labor or deliver a baby with low birth weight. They are also more likely to be iron deficient while they are breastfeeding. "
Iron deficiency was the most common micronutrient deficiency worldwide, with pregnant women and children under 5 years most at risk.
The condition was twice as common in New Zealand children aged 6 months to 2 years than in children of the same age living in Australia, Europe or the United States.
The frequency of iron deficiency among women of childbearing age had risen from 3 percent in 1997 to 7 percent in the most recent survey, conducted in 2008-2009.
To prevent negative effects on their childs early development, Grant recommended that pregnant women who drank large amounts of milk take iron supplements.
"An alternative would be to increase the nutrient content of milk consumed by mothers-to-be to keep both the mother and child healthy," he said.