Babies born alive by Caesarean section are almost three times more likely to die in the first month than those delivered naturally.
The finding, from a huge US study examining the risks of Caesarean births, will fuel the debate in the UK, where women are increasingly demanding the right to be given a choiceover the method of delivery.
However, the study did not include stillbirths, which are higher among vaginal deliveries, and looked only at what happened to babies after they were born. Had stillbirths been included, the risks of the two methods would have been closer.
Caesarean rates have soared worldwide in recent decades as techniques of surgery and anaesthesia have become safer. In the UK the rate has more than doubled from 9 per cent in 1980 to 22.3 per cent, more than one in five of all births, according to the Royal College of Obstetricians.
The rise has been driven by increasing medicalisationof birth, fear of litigation and the growing demand from some women for a convenient, pain-free form of delivery. Celebrities including Gwyneth Paltrow, Victoria Beckham and Stella McCartney, who have had Caesareans, have added to the trend.
There is a perception that pregnant career women with busy lives book their Caesareans in advance to fit in with their tight home and work schedules, but research has found no difference in the Caesarean rate among the top four social classes.
The latest study of more than 6 million babies born alive in the US between 1998 and 2001 found the death rate within 28 days among those delivered by Caesarean was 1.77 per 1,000, compared with 0.62 per thousand for babies born vaginally, a near threefold difference.
Because the risks of both types of delivery are extremely low, the difference amounts to 358 excess deaths among the 311,000 Caesarean births.
The researchers from the US Centres for Disease Control and Prevention selected only low-risk women for the study where there was "no [medically] indicated risk" for a Caesarean. These are elective - planned - Caesareans which may be carried out at the request of the motherand have risen by almost half in the US between 1996 and 2001, the researchers say.
Even after adjusting the results for social and medical differences among the women, babies born by Caesarean were still at more than twice the risk of dying in the first month of life.
Writing in the journal Birth, the authors say one explanation could be that labour stimulates respiration and prepares babies for birth. Marian MacDorman, who led the study, said: "Labour is an important part of the birthing process because it gets infants ready to breathe air and function outside the womb."
She added: "The point of this article is not to alarm anyone. Women considering an elective Caesarean obviously should consult with their doctor and this would be one piece of information that hopefully they could discuss."
Professor James Walker, head of obstetrics at St James's University Hospital, Leeds, and a spokesman for the Royal College, said the timing of Caesareans was crucial. "We know that if you deliver a baby at 37 or 38 weeks there is a small but significant risk of respiratory problems and death that is higher than at 39 weeks. That is why we wait till after 39 weeks if at all possible. This study confirms what is already known."