Stressed-out women are 12 per cent less likely to fall pregnant during their fertile time than those who are calm, according to Oxford University researchers.

While women have long been told to relax when trying for a baby, the evidence behind such claims was largely been anecdotal.

Now, the scientists have shown the importance of relaxation when it comes to falling pregnant.

They carried out saliva tests and analysed levels of the stress hormone cortisol in women and how that impacted on the likelihood of conceiving.

The authors concluded: “Stress significantly reduced the probability of conception each day during the fertile window.”

All the women were aged between 18 and 40 and were trying to get pregnant naturally.

Dr Cecilia Pyper, from the university’s National Perinatal Epidemiology Unit, 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.”