WOMEN who are stressed out while trying for a baby could be more likely to have girls, research suggests.
In the first study of its kind, experts found that high levels of the stress hormone cortisol were associated with the birth of more girls than boys.
Some 338 women from around the UK who were trying to get pregnant kept diaries about their lives, relationships and sex lives, and completed questionnaires about how stressed they felt.
Levels of cortisol and the enzyme alpha-amylase (an indicator of adrenaline) were also measured on day six of the women's monthly cycle for a period of up to six months, or until they fell pregnant.
Cortisol is linked to longer-term chronic stress, such as ill health, a demanding job or money worries.
Adrenaline, frequently called the "fight or flight" hormone, is linked to short-term stress.
During the study, 61pc (207 women) became pregnant. Of the babies born, 58 were boys and 72 were girls, indicative of a "strong female excess", the researchers said.
The difference in the sexes was not significant overall -- but was significant in the 50pc of women with the highest levels of cortisol.
The research showed they were up to 75pc less likely to have a boy.
No link was found with alpha-amylase.
Dr Cecilia Pyper, from the Department of Public Health at Oxford University, who worked on the study, said: "It is increasingly being recognised that long-term stress may be caused by financial difficulties.
"Women get very stressed when they haven't got enough money to care for their families, they worry on a day to day basis about how they are going to find the next bit of money to pay the next bill."
Previous research by the same team, which includes experts from the National Institutes of Health in the US, found stressed women are less likely to fall pregnant during their fertile time than those who are calm.
Other studies have revealed that stressful disasters (either natural or man-made) can upset the gender ratio.
It is unclear exactly why women with the highest levels of cortisol before pregnancy were more likely to have girls than boys.
The experts say more research is needed to see if the link between stress and sex ratio is genuine. It is already known that anxiety and stress in pregnant women may cause problems during pregnancy and with development of the baby.
Dr Allan Pacey, senior lecturer from the academic unit of reproductive and developmental medicine at the University of Sheffield, said: "We have known for a long time that some environmental factors, such as war, natural disasters and also occupation can affect the sex ratio at birth.
"It is perhaps no surprise, therefore, that pre-conception stress is correlated with the secondary sex ratio."