The babies of obese women are almost twice as likely to die in the womb or the first year of life as those of women who have a healthy weight.
Newcastle University researchers found that among women with a body mass index (BMI) greater than 30 in early pregnancy - which classified them as obese - 16 in 1,000 babies suffered foetal or infant death. Among those with a healthy BMI, between 18.5 and 24.5, only nine per 1,000 died.
However, babies of underweight mothers were also more likely to die.
Dr Ruth Bell, of the university's Institute of Health and Society, said that helping women achieve a healthy weight before pregnancy would "give the baby the best possible start in life".
The study is published in the journal Human Reproduction.