The study found women who are stressed out while trying for a baby could be more likely to have girls.
Experts found high levels of the stress hormone cortisol were associated with the birth of more girls than boys during a study of 338 women around the UK.
Researchers asked the female volunteers, who were trying to become pregnant, to keep diaries about their lives, relationships and sexual activity. They were also asked in the questionnaires about how stressed they felt.
Levels of cortisol and the enzyme alpha-amylase (an indicator of adrenalin) were also measured on day six of the women’s monthly cycles for a period of up to six months, or until they became pregnant.
The research found the 50% of women with the highest levels of cortisol were up to 75% less likely to have a boy. Cortisol is linked to longer-term chronic stress, such as ill health, a demanding job or money worries. Adrenalin, frequently called the “fight or flight” hormone, is linked to short-term stress.
Dr Allan Pacey, senior lecturer from the academic unit of reproductive and developmental medicine at Sheffield University, 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.
“In many animal populations we know the sex ratio can be altered in response to factors such as the availability of food and the probability of survival in difficult environments. It is perhaps no surprise, therefore, that pre-conception stress is correlated with the secondary sex ratio.”
During the study 61% (207) of women became pregnant. Of the babies born, 58 were boys and 72 were girls.
However, it is still unclear exactly why women with the highest levels of cortisol before pregnancy were more likely to have girls than boys.
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 become pregnant during their fertile time than those who are calm.
Dr Cecilia Pyper, from the Department of Public Health at Oxford University, who worked on the study, said: “Chronic stress can be related to many different stressful situations including financial pressures, stress at work, difficult relationships and problems related to physical or mental health problems.
“Long-term stress in women may be related to stress in the workplace or to stress in the home -- or it may be related to long-term emotional stress due to relationship difficulties.”
It is already known anxiety and stress in pregnant women may cause problems during pregnancy and with the development of the baby.
Dr Pyper added: “Women who are trying to conceive are already told how important it is for their future baby to take folic acid tablets, to stop smoking and to check they are immune to rubella. If the findings of this study are confirmed by larger studies, women may also be advised about reducing stress.”
However, despite the research findings, Dr Pacey said there was “no surefire way to influence the sex of a baby”.
He added: “Nature has a wonderful way of balancing everything out. So although we often see observations like this, on the whole there is generally the same number of boys born as there are girls.”