The problem is caused by hormones probably transferred from anxious mothers through the placenta.
The study shows for the first time ‘a direct, chemical link’ between stress in the mum-to-be and the child, says the Royal College of Obstetricians and Gynaecologists.
It advises pregnant women to avoid sustained or repeated stressful situations as far as possible.
Researchers measured levels of the stress hormone, cortisol, in 267 pregnant women. They also took blood tests from the mother and samples of amniotic fluid from around the baby.
Amniotic fluid, which is mostly produced by the baby, is a good indicator of the kinds of hormones and substances to which it has been exposed.
After 17 weeks of pregnancy, when cortisol levels rose in the mother’s blood, they also shot up in the amniotic fluid, the researchers found. The link between the two became stronger as the pregnancy advanced.