In the first of its kind, a study found high levels of the stress hormone cortisol were associated with the birth of more girls than boys.
Some 338 British women who were trying to get pregnant recorded how stressed they felt.
Levels of cortisol and the enzyme alpha-amylase – an indicator of adrenalin – were measured for up to six months or until they fell pregnant. Cortisol is linked to long-term stress, while adrenalin is linked to short-term stress.
A team from the Department of Public Health at Oxford University found women with the highest cortisol levels were up to 75 per cent less likely to have a boy. No link was found with alpha-amylase.
The experts say more research is needed to see if the link between stress and sex ratio is genuine.