An investigation of caesarean births, which are increasing worldwide, has revealed that dangers to both mother and baby are increased in hospitals where the procedure is most commonly carried out.
Mothers were more likely to require antibiotic treatment and to suffer illness after a caesarean while their babies were more likely to be delivered prematurely. Deaths were increased in mothers and babies.
The findings will fuel the debate over caesareans in Britain where women are increasingly demanding the right to choose their method of delivery. Campaigners say that as mothers are getting older and babies are bigger, the risks of natural birth have increased.
In Britain, the rate of caesarean births has more than doubled in 20 years to more than one in five (22 per cent) of all births. It has been driven by fear of litigation, the increasing medicalisation of childbirth and the demand from some women for a convenient, pain-free method of delivery.
The actress Gwyneth Paltrow, who is married to the Coldplay singer, Chris Martin, opted for a caesarean for the delivery of her second child, Moses, last month after a 70-hour labour with her first child, Apple.
Ms Paltrow, who lives in London, had Moses in New York but other celebrities have chosen the private Portland Hospital in the capital for caesarean deliveries including Victoria Beckham, Stella McCartney and Claudia Schiffer.
The latest research was conducted in South America where caesarean rates are higher than the UK, at an average of 33 per cent of all births. Well-heeled Latin American women are more anxious about the potential effect of a vaginal birth on their sex lives than British women and in some private hospitals the rate is up to 60 per cent.
Writing in The Lancet, the researchers say the study of 97,000 deliveries in eight countries in Latin America showed the risks to the baby began to increase as caesarean rates rose from 10 to 20 per cent. Just under half were "elective" or planned caesareans, while the remainder were emergencies carried out during labour when the women got into difficulties. They say their findings "should be generalisable beyond the participating institutions."
Jose Villar and colleagues from the World Health Organisation say the number of caesareans performed "reflect a complex social process, affected by clinical status, family and social pressures, the legal system, availability of technology, women's role models".
They add: "High rates of caesarean delivery do not necessarily indicate good quality care or services. Our results show how a medical intervention that is effective when applied to sick individuals in emergency situations can do more harm than good when applied to healthy populations."
In Britain, the rise in the caesarean rate has been most severely criticised by the Royal College of Midwives which says it is depriving women of the experience of a natural birth. In 2004, the National Institute for Clinical Excellence (Nice) launched guidelines aimed at curbing the rise.
A caesarean costs about £3,200 compared with £1,700 for a vaginal birth. The evidence submitted to NICE showed that women who had a caesarean were slightly less likely to suffer incontinence or prolapse of the womb but more likely to suffer thrombosis - blood clots in the legs or lungs - bladder injury or to require further surgery.
Babies born by caesarean were more likely to have breathing problems at birth.