A new study casts doubt on revised Institute of Medicine guidelines for women's weight gain during pregnancy for preventing large-for-gestational-age babies. This large, prospective study published online in Obstetrics & Gynecology found a positive association between a mother’s fat-free mass -- or, her total body mass minus the fat -- with increased birth weight, while no link was found between maternal fat mass and birth weight. The researchers evaluated birth outcomes for 2,618 pregnant women whose body composition was measured in the first trimester using multifrequency segmental bioelectric impedance analysis. Previous evidence linking maternal obesity -- which is usually determined by body mass index (BMI) -- with increased birth weight is problematic, say the authors, because BMI does not measure distribution of fat or fat-free mass. The findings are important, conclude the authors, because in 2009, the IOM lowered its recommended gestational weight gain for obese women to 5-9 kilograms due to concerns that increasing maternal obesity rates would result in more high birth-weight babies. The authors note that previously shown links between maternal obesity and large-for-gestational-age babies may be the result of gestational diabetes, rather than maternal obesity itself. Read the abstract.