Babies who undergo a difficult birth and are delivered using forceps are more likely to develop problems such as aggression during childhood compared with those born by Caesarean section, according to a study in China.

The researchers believe the behavioural problems may be linked to high levels of cortisol, a hormone the body produces during a stressful and difficult birth.

"The association between mode of delivery and subsequent childhood psychopathology is possibly related to cortisol response," they write in a paper published in BJOG: An International Journal of Obstetrics and Gynaecology.

Previous studies have found that cortisol levels in cord blood are lowest in babies born by elective Caesarean section, followed by spontaneous vaginal delivery.

The highest levels of cortisol are found in those born by assisted vaginal delivery using forceps or vacuum extraction.

"Cortisol levels have been linked to childhood psychopathology, however, more studies are still needed to look at this in more detail," say the scientists, led by Professor Jianmeng Liu, deputy director of the Institute of Reproductive and Child Health at the Peking University Health Science Centre.

The study involved 4,190 children who were born in Zhejiang and Jiangsu, and they were assessed between the age of four and six for problems such as being withdrawn, anxious, depressed, attention difficulties, and delinquent and aggressive behaviour.

Such problems were lowest in children delivered by Caesarean section and highest in those delivered using instruments such as forceps and vacuum, the researchers say.

Caesarean section births are increasing, particularly in the richer southeastern parts of the country where rates have risen to 56 per cent in 2006 from 22 per cent in 1994.

Caesarean section delivery on request by mothers is a leading contributor to this trend.

The method accounted for 3.6 per cent of all Caesarean section births in 1994 and 36 per cent in 2006 in southeast China.