Women have been warned against eating for two when they are pregnant.
Women are advised to eat sensibly during pregnancy
Doctors from the Karolinska Institute in Sweden say women who put on too much weight when they are expecting risk becoming obese later in life.
Speaking at the European Congress on Obesity, they said those who put on 16kg or more were most at risk.
Obesity can increase the risk of heart disease, diabetes and cancer. The warning followed a study of more than 2,300 women over 15 years.
The doctors found that women who gained less than 16kg were on average 1.4kg heavier one year after the birth.
Those who had put on more were on average 5.5kg heavier a year later. Fifteen years down the line, these women weighed 17kg heavier on average.
Dr Yvonne Linne, one of those involved in the study, told the Congress in Prague that women who put on between 12kg and 14kg during pregnancy fared best.
Overall, the number of women who were obese jumped from 7% at the beginning of the study to 12% a year after the birth and then to 31% by 15 years.
Dr Linne said many women expected to lose their excess weight after giving birth, particularly as a result of breastfeeding. But she said most failed to lose weight.
She urged women to eat sensibly during pregnancy.
"Women should try to maintain the lifestyle they had before the pregnancy and go back to their usual habits," she said.
The National Childbirth Trust recently published a book with information on what and how women should eat during pregnancy.
"We all know that putting on too much weight can be a problem," said Belinda Phipps, its chief executive.
"Putting a lot of weight on during pregnancy can also complicate the pregnancy.
"Our advice to women is not to eat for two during pregnancy, but to eat for their appetite."