Caesarean birth greatly increases a baby's chances of developing allergies, a study has found.
Infants delivered by C-section are five times more likely than those born naturally to become allergic to common triggers such as dust mites and pets, according to the research.
Scientists believe the babies are left vulnerable by avoiding the journey through the birth canal, which would normally expose them to their mother's bacteria.
The discovery lends support to the “hygiene hypothesis” that links childhood allergy to over-clean conditions early in life.
Researcher Dr Christine Cole Johnson, from the Henry Ford Hospital in Detroit, US, said: “This further advances the hygiene hypothesis that early childhood exposure to micro-organisms affects the immune system's development and onset of allergies.
“We believe a baby's exposure to bacteria in the birth canal is a major influence on their immune system.”