Lack of iodine can lead to reduced mental development in their children, the study found.
Iodine is essential for producing the hormones made by the thyroid gland, which have a direct effect on foetal brain development.
Adults require 150mg of iodine a day – this rises to 250mg while pregnant or breast-feeding. 150g of yogurt provides about 100mg iodine, whereas 1 egg contains about 20mg.
A group of researchers from Surrey and Bristol universities, led by Professor Margaret Rayman of the University of Surrey, measured the iodine concentration in urine samples taken in the first trimester from 1,040 pregnant women, classifying 67% as being iodine deficient.
Mental development of their children was assessed by measuring child IQ at age eight, and reading ability at nine.
Adjusting the results for external factors likely to affect these scores, such as parental education and breast-feeding, the researchers found that children of women in the iodine-deficient group were significantly more likely to have low scores of verbal IQ, reading accuracy and reading comprehension.
The lower the mother's concentration of iodine, the lower the average scores for IQ and reading ability in the children."
Prof Rayman said: "Our results clearly show the importance of adequate iodine status during early pregnancy."
Dr Sarah Bath, a co-author and registered dietician, said: "Pregnant women and those planning a pregnancy should ensure adequate iodine intake; good dietary sources are milk, dairy products and fish.
"Kelp supplements should be avoided as they may have excessive levels of iodine."
For more information on iodine-rich foods, download the British Dietetic Association factsheet.