For years, maternal health experts have worried about a troubling statistic: More than half of all pregnant women in America are overweight or obese when they conceive, putting them and their children at a higher risk of developing diabetes and other health problems.

So about a decade ago, the federal government launched a multimillion-dollar trial to see whether diet and exercise could help overweight women maintain a healthy weight during their pregnancies and potentially reduce their rate of complications. On Thursday , the findings were announced, and the results were mixed: Starting a diet and exercise program around the beginning of their second trimesters helped many women avoid excess weight gain during their pregnancies. But it did not lower their rate of gestational diabetes, hypertension and other adverse outcomes.

Experts said the research was both encouraging and sobering. It confirmed that overweight and obese women can safely limit their pregnancy weight gain with lifestyle interventions. But it also suggests that to improve obstetric outcomes and the health of their babies, women who are carrying extra weight may need to make significant lifestyle changes before they conceive, said Dr. Alan Peaceman, the chief of maternal fetal medicine at Northwestern University Feinberg School of Medicine and the lead investigator of the study, which was published in Obesity .

“This is a problem that is more important now than it’s ever been, and it needs to be addressed,” he added. “We are going to have to start talking to women who are overweight or obese even before pregnancy and explain to them the risk of that weight on a potential pregnancy.”