Women often say during pregnancy that they are 'eating for two'.
But experts have warned overeating during this crucial period can increase the risk of the baby later developing health problems.
Women who gain too much weight during pregnancy run the risk of giving their offspring health problems in later life, researchers found.
Babies born to mothers who were overweight during their late pregnancy were more likely to develop illnesses such as heart disease, type 2 diabetes and stroke, experts said.
Instead, pregnant women should ensure they lead a healthy lifestyle, with even the period just before birth being 'fundamental' to giving their children the best chance.
Experts have warned against becoming overweight during pregnancy, as it can increase the risk of the baby developing health problems including depression, heart disease, type 2 diabetes and stroke in later life
The findings are published as part of a major European obesity project, which concluded that strategies must urgently be developed to prevent obesity in girls and in women of childbearing age.
The importance of a healthy diet during pregnancy was highlighted by the project.
As part of the study, researchers followed more than 13,000 people from birth in the 1930s and 40s until their present age.
They found the placenta of mothers eating a high-fat diet offered weakened protection to the foetus against the stress hormone cortisol.
This can mean that the growth of the foetus is reduced and these offspring are more likely to suffer mood disorders in adulthood, researchers at the University of Edinburgh found.
Other key findings related to protective caps on the end of DNA which are known as telomeres.
Having long telomeres protects the DNA to enable it to function and repair, while shorter telomeres are markers of disease and a shorter lifespan, scientists said.
The team also discovered that obesity, combined with high weight gain during pregnancy, is associated with shorter telomere length in women's offspring and as they get older.
Dr Patricia Iozzo, of the Institute of Clinical Physiology, in Pisa, Italy, who led the project, said that even if a pregnant woman was born with shorter telomeres herself and has a high body mass index (BMI) she can reverse the situation by being physically active and produce healthier children.
She said: 'The project has underlined the importance of preventing obesity in pregnancy, preventing excess weight gain during pregnancy, and also maintaining healthy diet without too much fat - all of which can have short and long term effects on the health of the mother and her child.
Overweight mothers had babies with shorter telomeres - which are protective 'caps' on DNA. Having shorter telomeres is associated with higher risk of disease and a shorter lifespan
'In the context of maternal-offspring health, attention should be devoted to the prevention of overweight and obesity among young girls, representing 'tomorrow's mothers'.
Targeted strategies are also needed to ensure pregnant women do not add excess weight and protect their physical and mental health, and that of their children.'
Louise Silverton, director for midwifery at the Royal College of Midwives, said women should try to be an ideal weight before they become pregnant.
She said: 'This report adds more to our knowledge about the importance of the environment in which the foetus is nurtured.
'After birth, women need support to develop healthy patterns of eating and exercise for themselves and their family.
'For women who are overweight or obese they need support and signposting to access weight-loss services to ensure that they are an ideal weight before they embark on their next pregnancy.'
The Developmental ORIgins of healthy and unhealthy AgeiNg: the role of maternal obesity (Dorian) consortium, as it was known, was funded by the European Commission’s Framework Programme.
Read more: http://www.dailymail.co.uk/health/article-2955930/Why-pregnant-women-SHOULDN-T-eat-two.html#ixzz3S1bLTI4L
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