Lead author Rika Tanda, a doctoral candidate in nursing at Ohio State University, used data from the National Longitudinal Survey of Youth 1979 Mother and Child Survey, a nationally representative sample of men and women ages 14-21 in December 1978.
Tanda collected information on 3,412 children born to the women who were studied.
The researchers gauged the children's cognitive function based on their performance on Peabody Individual Achievement Test reading recognition and math assessments. They then calculated the mothers' body mass index based on their reported heights and weights.
More than half of mothers had normal BMI before pregnancy, and 9.6 percent were obese -- with a BMI of 30 or higher.
The study, published online in Maternal and Child Health Journal, found maternal prepregnancy obesity was negatively associated with math and reading test scores. Children of obese women scored, on average, three points lower on reading and two points lower on math than did children of healthy-weight women, the study said.
Although the score differences seem small, the effects of prepregnancy obesity were equivalent to a seven-year decrease in the mothers' education and significantly lower family income, two well-known risk factors that negatively affect childhood cognitive function, Tanda said.