Mothers who put on or fail to shift excess weight after having a baby risk problems in later pregnancies, experts have warned.
Pregnant women should maintain a healthy weight
A few kilos, even among women who are not overweight, can make all the difference, The Lancet study shows.
Modest weight gain after a first child increased the risk of high blood pressure, diabetes and stillbirth during the women's second pregnancies.
Experts say the findings are worrying given rising obesity rates.
Based on current trends, two-thirds of women will be overweight or obese by 2010.
Doctors already know that these women can find it harder to conceive and risk complicated pregnancies.
The Swedish study, involving 150,000 women, found an increase in body mass index (BMI) - a measure of obesity - by only one to two units was enough to increase the risk of pregnancy complications.
An ideal BMI (weight in kilograms divided by height in metres squared) is anything between 18.5 and 25.
A woman who is 5ft 5ins tall and weighs 139 pounds has a BMI of 23.
If she gained 6.6lbs, her BMI would increase by one unit to 24.
In the study, women who gained one to two BMI units over an average of two years between having their first baby and becoming pregnant with their second child increased their risk of pregnancy-related diabetes and high blood pressure by 20-40%.
The more weight gained, the higher the risk of complications was.
A gain of three or more BMI units increased also the risk of the baby being born dead.
Co-author Dr Eduardo Villamor, from the Harvard School of Public Health in Boston, US, said: "Women do not need to become overweight or obese in order to increase their chances of having poor gestational outcomes.
"A relatively modest increase in weight between pregnancies could lead to serious illnesses."
Weight loss struggle
But this also means that a small amount of weight loss can make a big difference, particularly among women who are overweight, he said.
Professor Adam Balem of the Royal College of Obstetricians and Gynaecologists said: "The results are very striking. Obesity is an everyday problem we see in clinics.
"Women do find it harder to lose weight after pregnancy."
He said factors like whether a woman breastfeeds, the type of labour she has and how long it takes her to become active again afterwards all play a part.
"And as you get older you have to eat less and exercise more to keep the weight off, which can be hard for busy women with young children.
"The best advice is for women to try and breastfeed and do your best to maintain a level of fitness and return to your pre-pregnancy weight before embarking on another pregnancy."
Gillian Fletcher of the National Childbirth Trust said: "This is something we have to take notice of, but we have to be cautious about the messages we give.
"So many women try to lose weight postnatally and may well go the other way and lose too much."
She said crash dieting was dangerous, especially for women who were breastfeeding or trying to become pregnant.