An earlier study also found the procedure puts the baby at greater risk, too. A baby born by Caesarean section is three times more likely to die soon afterwards compared with the one born naturally.
The dangers of Caesarean delivery came to light when researchers from Oxford University tracked the progress of 97,095 births from health facilities throughout Latin America.
The operation was only less dangerous for the mother when the baby was in breech position, and it was considered a necessary intervention.
But many of the procedures that went wrong for the mother or baby were elective; in other words, the mother decided to have the operation even though there was no medical reason to do so.
The World Health Organization reckons that the rate of Caesarean sections should never be higher than 10 per cent of all births; that should be sufficient to cover any complications, they estimate.
In fact, around 30 per cent of all births in the USA - and the figure is only slightly lower for the UK - are by Caesarean section. In Latin America, the rate is the highest in the world at 33 per cent.
(Source: British Medical Journal, 2007; 335: 1025-9).