British research confirms that women should relax if they want to fall pregnant.
Those anxious during their fertile time are less likely to conceive than those who were calm. Up till now the evidence behind this belief had largely been anecdotal.
Now, scientists from Oxford University have found stressed women were 12 per cent less likely to conceive.
They carried out saliva tests on 274 women and analysed levels of the stress hormone cortisol and the enzyme alpha-amylase (an indicator of adrenalin levels).
All the women were aged between 18 and 40 and were trying for a baby naturally.
None of the women had not been trying for more than three cycles prior to the start of the study and had not undergone fertility treatment.
Researchers carried out the tests on day six of each woman's menstrual cycle for a total of six cycles or until the woman fell pregnant.
They used fertility monitors to identify ovulation and confirmed the pregnancies with testing kits.
The study, published in the journal Fertility and Sterility, found no effect from cortisol on the chances of falling pregnant.
But women in the group with the highest levels of alpha-amylase had a 12 per cent lower chance of falling pregnant for each day of their most fertile days than those with the lowest levels of alpha-amylase.
The authors concluded: 'Stress significantly reduced the probability of conception each day during the fertile window.'
Dr Cecilia Pyper, from the National Perinatal Epidemiology Unit at the Oxford University, said: 'This is the first study to find that a biological measure of stress is associated with a woman's chances of becoming pregnant that month.
'We found that those women with high levels of a marker for stress were less likely to succeed in conceiving.
'The findings support the idea that couples should aim to stay as relaxed as they can about trying for a baby.
'In some people's cases, it might be relevant to look at relaxation techniques, counselling and even approaches like yoga and meditation.
'Many couples are very keen to know what they should do to improve their chances of conceiving and having a healthy baby, and this will help us provide the best advice.'
Dr Pyper said more research was needed on stress and pregnancy, and called for a larger study.
'More work is required to understand the size of the effect of stress on the chances of becoming pregnant and how it compares to the effects of factors like smoking, obesity and alcohol,' she said.
'Further studies would also be needed to determine whether relaxation and stress-reduction techniques could have beneficial effects and improve couples' chances of conceiving.'
Experts from the National Institute for Child Health and Human Development in the US also worked on the study.
Dr Germaine Buck Louis, from the unit, said more research was needed to see if delays in conceiving raise women's stress levels, thereby further reducing their chances of falling pregnant