Women who gain more than 35 pounds during pregnancy have a higher risk of becoming overweight or obese, Swedish scientists said Friday.

Expectant mothers put on an average of about 26 pounds and most of it disappears in the first year after birth.

Dr. Yvonne Linne, an obesity specialist at the Karolinska University Hospital in Stockholm, told a medical conference here that 26-30 pounds is fine but gaining more can cause problems.

“Over 16 kilos (35 pounds) you are at risk of retaining the weight and starting a trend toward obesity,” she said.

Linne stressed that women should not diet during pregnancy or restrict their food intake but should eat sensibly.

She and her colleagues monitored the weight gain of 2,342 pregnant women in Stockholm and followed up their progress at one year and 15 years later.

Women who had gained less than 35 pounds were about three pounds heavier a year after the birth but those who put on more weight had retained 12 pounds at one year and weighed 37 pounds more 15 years later.

“Pregnancy is the only lifetime event where you are allowed to gain weight,” said Linne.

She added that some women put on too much weight and think it will easily disappear during breastfeeding, but it usually doesn’t happen. “Our study showed there is no effect of breastfeeding on weight development,” said Linne.

Research has also shown that weight gain is cumulative with each pregnancy and is highest between the fifth and sixth pregnancy, although most modern families are not that large.

About seven percent of the women at the start of the study were overweight. The number increased to 12 percent a year after the birth and rose to 31 percent by 15 years.

Women who were overweight or obese before the pregnancy maintained their increased weight after the birth.

“Women should try to maintain the lifestyle they had before the pregnancy and go back to their usual habits,” she added.

Linne and her colleagues are now studying the children of the women who took part in the study to determine whether weight gain during pregnancy has an impact on them.

About 2,500 doctors, scientists and obesity experts are attending the four-day European Congress on Obesity.