Many babies of obese women with gestational diabetes are already too big by the time moms are diagnosed with the condition.
Women are typically screened for gestational diabetes at around 28 weeks or six months of pregnancy.
But researchers found babies of obese and diabetic moms were five times more likely to show excessive growth at the time of this test.
In fact, the size of the fetus was already abnormal by 20 weeks of pregnancy.
Experts say earlier screenings could improve the outcomes for these women and their babies.
Offspring of women with gestational diabetes are at an increased risk of childhood obesity.