It's well established that being overweight during pregnancy can have negative consequences for both a woman's own health and that of her baby. The NHS advises women to lose weight before becoming pregnant to avoid complications during pregnancy and reduce the baby's risk of developing chronic diseases.And new research points to another reason for watching your weight while pregnant.Babies born to mothers who are obese are up to 17 years "older" biologically than they should be, which could put them at greater risk of developing chronic illnesses in later life, according to scientists writing in the journal BMC Medicine The study analysed newborns' telomeres, the protective structures at the end of chromosomes, as a marker of "biological age". Telomeres get shorter as we age and because of unhealthy habits, which leaves our DNA more at risk of becoming damaged when our cells divide.It is the first time the link between a newborn's telomeres and his or her mother's weight has been studied, reported The Times The researchers analysed blood samples from 743 mothers and their babies and found a correlation between the mothers' pregnancy weight and the length of their babies' telomeres.With each one-point increase in the woman's BMI, the results showed the babies' telomeres were 50 DNA pairs shorter – equal to what people lose in one and a half years of adult life.Previous research has linked shorter telomeres to a greater risk of heart disease , as well as diabetes and earlier death.“This illustrates the public health significance of our findings, as newborns from obese mothers compared with newborns from normal weight mothers were biologically approximately 12 to 17 years older, based on telomeric year equivalence in adulthood,” wrote the scientists.Some said the findings lend weight to the existing advice that women take care of their health during pregnancy.“This intriguing study provides further evidence of the lifelong impact of maternal obesity on a child’s life," said Professor Neena Modi, president of the Royal College of Paediatrics and Child Health, The Times reported She said the study highlights how "babies born to obese mothers may be at greater vulnerability to chronic diseases in adult life", adding that it "provides a strong justification for intervention in pregnancy, infancy, childhood and young adult life to tackle the national burden of obesity".