Roughly half of first-time mothers in the U.S. gain too much weight during pregnancy, about the same proportion who are overweight at the start, a study finds.

Researchers compared data on more than 2 million U.S. women to a group of about 1,650 of their peers in Germany, where almost three-fourths of first-time mothers started pregnancy at a healthy weight and only 27 percent exceeded weight gain recommendations.

“The differences in overweight rates pre-pregnancy and in weight gain are surprising,” lead study author Dr. Joachim Dudenhausen, an obstetrics researcher at Charite University Medicine in Berlin, said by email. “We must follow these data for some years to find an explanation.”

While the cause of the discrepancies is unclear, the health effects of excessive weight gain during pregnancy, or starting out overweight, are well known, he said.

Short-term problems can include overweight babies, births complicated by infants’ shoulders getting stuck on the way out, and a greater likelihood of a surgical delivery by cesarean section, he said.

Over the long haul, babies can have a greater risk of diabetes, hypertension and metabolic syndrome, which increases the odds of heart attacks and strokes in adulthood.

Women who are underweight at the start of pregnancy should gain 28 to 40 pounds, while women who are normal weight are advised to gain 25 to 35 pounds, according to the U.S. Institute of Medicine.

For overweight women, a 15- to 25-pound gain is recommended and obese women should gain just 11 to 20 pounds.

To see how often women were below, within or above recommended weight ranges for pregnancy, Dudenhausen and colleagues assessed weight gains for the German and American pregnancies based on their pre-pregnancy body mass index (BMI), a measure of weight relative to height.

An adult who is 5 feet 9 inches tall and weighs from 125 to 168 pounds would have a BMI of 18.5 to 24.9 and be considered a healthy weight, according to the U.S. Centers for Disease Control and Prevention. An obese adult at that height would have a BMI of 30 or more and weigh at least 203 pounds.

Pre-pregnancy, 72 percent of the German women were at a healthy weight, compared with just 47 percent for the Americans, the study found. Four percent of the women in the U.S. and in Germany were underweight.

In Germany, 39 percent of women gained too little during pregnancy, almost twice the rate in the U.S.

And, overall, less than one in three American or German women stayed within the recommended weight gains during pregnancy.

The findings confirm that obesity is an epidemic, and that doctors need to counsel women on proper nutrition and exercise well before pregnancy to make it more likely that they start out at a healthy weight, the researchers write in American Journal of Obstetrics and Gynecology.

“Over the last two to three decades, there has been a significant increase in obesity among pregnant women in the U.S., and the proportion of overweight women in Germany is significantly less but also increasing,” said Dr. Amos Grunebaum, director of obstetrics at NYP/Weill Cornell Medical Center in New York.

Obesity increases the risk of infertility and miscarriages, Grunebaum, a co-author on the study, said by email.

“Obese women may have to make significant changes in their eating habits in order to improve chances of getting pregnant and having healthy babies,” he said.