The experience of labour and childbirth can trigger post-traumatic stress symptoms more commonly associated with disasters and war, say scientists.

A study by psychologists at Sheffield University found that new mothers may experience nightmares and negative and disturbing thoughts and images.

The worst affected seem to be those with unplanned pregnancies and those not accompanied by partners at the birth.

Dr Pauline Slade and Dr Jo Czarnocka assessed 264 women within 72 hours of a "normal" birth.

When questioned again six weeks afterwards, one in four women reported experiencing either intrusive thoughts, images or nightmares, actively tried to avoid situations which triggered memories of labour, or showed excessively high levels of wariness.

All these symptoms are features of post-traumatic stress disorder.

However, only 3 per cent showed all these symptoms.

Important factors

The researchers also looked at what factors influenced the development of these symptoms.

They found the length of labour, the pain of birth and the type of birth did not seem to be important.

Women who were not accompanied by partners at the birth or who had unplanned pregnancy were more likely to suffer, as were those who had experienced prior mental health problems.

Women who felt that they had little control over the birth process were also vulnerable.

Those who were fearful during labour were particularly at risk.

Many of those affected said it had changed their plans about having any future children.

Dr Slade said: "These sorts of post-traumatic stress symptoms had been thought to be likely to occur when people had experienced events outside of the range of normal experience.

"It now seems that normal events such as labour may trigger such responses."

"We need to have an understanding that post-natal distress may be rather broader-based than we had previously thought."

The research will be published in the British Journal of Clinical Psychology.