A scientific study has shown for the first time that high stress levels may delay pregnancy.
Oxford University experts measured stress hormones in women planning a baby naturally and found the most stressed had a reduced chance of becoming pregnant.
Relaxation might help some couples, but more research is needed, they say.
The study, in the journal Fertility and Sterility, followed 274 healthy women aged 18-40 planning a pregnancy.
Age, smoking, obesity and alcohol are known to affect pregnancy success, but the influence of stress is less clear.
Markers for two stress hormones - adrenalin, the body's fight or flight hormone, and cortisol, connected with chronic stress - were measured in saliva.
Women with the highest levels of alpha-amylase (an indicator of adrenalin levels) had about a 12% reduced chance of getting pregnant during their fertile days that month compared with those with the lowest levels of the marker.
No difference in the chance of becoming pregnant was found with cortisol.
Anecdotal reports have long linked stress with infertility, but direct scientific evidence has been hard to find.
Dr Cecilia Pyper, of the National Perinatal Epidemiology Unit at the University of Oxford, said their study aimed to improve understanding of the factors that influence pregnancy in normal healthy women.
She 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.
"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."
The research was carried out in collaboration with the Eunice Kennedy Shriver National Institute for Child Health and Human Development in the US.
It is part of a larger study looking at the effect of factors like smoking, alcohol, and caffeine on chances of pregnancy.
Commenting on the study, Joanne Taylor, midwife for baby charity Tommy's said: "Stress does affect people in different ways, and therefore may affect some women's chances of trying to conceive.
"There are a number of ways in which women can prepare themselves for a healthy pregnancy - maintaining a healthy diet, taking folic acid supplements and keeping stress levels to a minimum is a good start."