The researchers have also said that they have not found any correlation between maternal thyroid function and newborn thyroid function in a Boston-area sample.

While, normal thyroid function is vital for healthy brain development, it has been suggested in earlier studies that even mild maternal hypothyroidism during pregnancy may have adverse effect on child cognitive development.

In the Project Viva, a team of researchers led by Dr. Emily Oken of Harvard Medical School and Harvard Pilgrim Health Care, studied 500 children born 1999-2003 to evaluate the relationship between thyroxine levels in newborns, first trimester maternal thyroid function, and childhood cognition.

In the first place, scientists tested mothers' thyroid stimulating hormone, thyroxine, and thyroid peroxidase antibody levels at an average of 10.2 weeks gestation and later measured newborns' thyroxine levels from whole blood samples after birth.

Later, they performed cognitive testing when the infants were six months old using the visual recognition memory test, a measure of infant cognition that can predict later childhood IQ and specific abilities in perceptual speed, language, and memory.

At the age of three years, the children were tested with the Peabody Picture Vocabulary Test a measure of verbal ability or scholastic aptitude, and the Wide Range Assessment of Visual Motor Ability , which evaluates visual-spatial analysis, visual-motor ability, and fine motor skills.